You need a transformer. Check below the modified method :

public static String removeNameSpace(String xml) {


        try {
             TransformerFactory tf = TransformerFactory.newInstance();
             Transformer transformer = tf.newTransformer();
             transformer.setOutputProperty( OutputKeys.METHOD, "xml" );
             transformer.setOutputProperty( OutputKeys.INDENT, "false" );
            System.out.println("before xml = " + xml);
            DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
            DocumentBuilder builder = factory.newDocumentBuilder();
            InputSource inputSource = new InputSource(new StringReader(xml));
            Document xmlDoc = builder.parse(inputSource);
            Node root = xmlDoc.getDocumentElement();
            NodeList rootchildren = root.getChildNodes();
            Element newroot = xmlDoc.createElement(root.getNodeName());
            for (int i = 0; i < rootchildren.getLength(); i++) {
                newroot.appendChild(rootchildren.item(i).cloneNode(true));
            }
            xmlDoc.replaceChild(newroot, root);
            DOMSource requestXMLSource = new DOMSource( xmlDoc.getDocumentElement() );
            StringWriter requestXMLStringWriter = new StringWriter();
            StreamResult requestXMLStreamResult = new StreamResult( requestXMLStringWriter );            
            transformer.transform( requestXMLSource, requestXMLStreamResult );
            String modifiedRequestXML = requestXMLStringWriter.toString();

            return modifiedRequestXML;
        } catch (Exception e) {
            System.out.println("Could not parse message as xml: " + e.getMessage());
        }
        return "";
    }

Output :

before xml = <?xml version="1.0" encoding="UTF-8"?><Payment xmlns="http://api.com/schema/store/1.0"><Store>abc</Store></Payment>
afterNsRemoval = <?xml version="1.0" encoding="UTF-8"?><Payment><Store>abc</Store></Payment>

This is a fairly common question, and a quick search turned up the following questions:

How do I remove namespaces from xml, using java dom?

Remove namespace from XML in Java

Perssonally, I think XSLT is the most obvious technique because this is exactly what XSLT was invented for ( XML-to-XML tranformations). I have successfully used this XSLT to strip namespaces (credit goes to https://stackoverflow.com/users/18771/tomalak):

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="node()">
    <xsl:copy>
      <xsl:apply-templates select="node()|@*" />
    </xsl:copy>
  </xsl:template>

  <xsl:template match="*">
    <xsl:element name="{local-name()}">
      <xsl:apply-templates select="node()|@*" />
    </xsl:element>
  </xsl:template>

  <xsl:template match="@*">
    <xsl:attribute name="{local-name()}">
      <xsl:apply-templates select="node()|@*" />
    </xsl:attribute>
  </xsl:template>
</xsl:stylesheet>

You will find the Java code for executing that XSLT in both threads.

You can use xslt for that. Try

removeNs.xslt

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output method="xml" indent="yes" />

    <xsl:template match="*">
        <xsl:element name="{local-name(.)}">
            <xsl:apply-templates select="@* | node()" />
        </xsl:element>
    </xsl:template>
    <xsl:template match="@*">
        <xsl:attribute name="{local-name(.)}">
      <xsl:value-of select="." />
    </xsl:attribute>
    </xsl:template>
</xsl:stylesheet>

Sample.java

import java.io.File;

import javax.xml.transform.Source;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerConfigurationException;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.stream.StreamResult;
import javax.xml.transform.stream.StreamSource;

public class Sample {

    public static void main(String[] args) {
        try{
            TransformerFactory factory = TransformerFactory.newInstance();
        Source xslt = new StreamSource(new File("removeNs.xslt"));
        Transformer transformer = factory.newTransformer(xslt);

        Source text = new StreamSource(new File("data.xml"));
        transformer.transform(text, new StreamResult(new File("output.xml")));
        System.out.println("Done");
        } catch (TransformerConfigurationException e) {
            e.printStackTrace();
        } catch (TransformerException e) {
            e.printStackTrace();
        }
    }

}

Use the Regex function. This will solve this issue:

public static String removeXmlStringNamespaceAndPreamble(String xmlString) {
  return xmlString.replaceAll("(<\\?[^<]*\\?>)?", ""). /* remove preamble */
  replaceAll("xmlns.*?(\"|\').*?(\"|\')", "") /* remove xmlns declaration */
  .replaceAll("(<)(\\w+:)(.*?>)", "$1$3") /* remove opening tag prefix */
  .replaceAll("(</)(\\w+:)(.*?>)", "$1$3"); /* remove closing tags prefix */
}

I have similar requirements at the moment. The only solution that worked for me is implementing a wrapper for XMLStreamWriter.

Please, take a look at my answer here. I've also described there other solutions I tried out.

Serialization using code from the link above looks like this:

XMLOutputFactory factory = XMLOutputFactory.newFactory();

StringWriter writer = new StringWriter(XML_BUFFER_INITIAL_SIZE);
XMLStreamWriter xmlWriter = null;

try {
  xmlWriter = factory.createXMLStreamWriter(writer);
  JAXBContext context = JAXBContext.newInstance(MyJAXBGeneratedClass.class);
  Marshaller marshaller = context.createMarshaller();
  marshaller.marshal(reportContainer, new NamespaceStrippingXMLStreamWriter(xmlWriter));
  xmlWriter.flush();
}
finally {
  if (xmlWriter != null)
    xmlWriter.close();
}

return writer.toString();

I found a solution to my problem. Posting it here if somebody lands here searching for an answer. I took a different approach. I let the CXF create the SOAP message and then I used the CXF custom interceptor to make changes to the message elements.

Configuration XML:

<!-- CXF Bus Configuration -->

<cxf:bus name="clientBus">
    <cxf:outInterceptors>
        <bean class="com.xxx.xxx.xxx.xxx.CustomMessageInterceptor" />
    </cxf:outInterceptors>
    <cxf:features>
        <cxf:logging/>
    </cxf:features>
</cxf:bus> 

CustomMessageInterceptor.java

package com.xxx.xxx.xxx.xxx;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import org.apache.commons.io.IOUtils;
import org.apache.cxf.binding.soap.interceptor.SoapPreProtocolOutInterceptor;
import org.apache.cxf.io.CachedOutputStream;
import org.apache.cxf.message.Message;
import org.apache.cxf.phase.AbstractPhaseInterceptor;
import org.apache.cxf.phase.Phase;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Component;


@Component
public class CustomMessageInterceptor extends AbstractPhaseInterceptor<Message> {

    public CustomMessageInterceptor() {
        super(Phase.PRE_STREAM);
        addBefore(SoapPreProtocolOutInterceptor.class.getName());
    }

    private Logger log = LoggerFactory.getLogger(this.getClass());

    public void handleMessage(
            Message message) {

        OutputStream os = message.getContent(OutputStream.class);

        CachedStream cs = new CachedStream();
        message.setContent(OutputStream.class, cs);

        message.getInterceptorChain().doIntercept(message);

        try {
            cs.flush();
            IOUtils.closeQuietly(cs);
            CachedOutputStream csnew = (CachedOutputStream) message.getContent(OutputStream.class);

            String currentEnvelopeMessage = IOUtils.toString(csnew.getInputStream(), "UTF-8");
            csnew.flush();
            IOUtils.closeQuietly(csnew);

            if (log.isDebugEnabled()) {
                log.debug("Outbound message: " + currentEnvelopeMessage);
            }

            String res = changeOutboundMessage(currentEnvelopeMessage);
            if (res != null) {
                if (log.isDebugEnabled()) {
                    log.debug("Outbound message has been changed: " + res);
                }
            }
            res = res != null ? res : currentEnvelopeMessage;

            InputStream replaceInStream = IOUtils.toInputStream(res, "UTF-8");

            IOUtils.copy(replaceInStream, os);
            replaceInStream.close();
            IOUtils.closeQuietly(replaceInStream);

            os.flush();
            message.setContent(OutputStream.class, os);
            IOUtils.closeQuietly(os);

        }
        catch (IOException ioe) {
            log.error("Unable to perform change.", ioe);
            throw new RuntimeException(ioe);
        }
    }

    protected String changeOutboundMessage(
            String currentEnvelope) {
        currentEnvelope = currentEnvelope.replace("<ClruInsert xmlns=\"\" xmlns:ns22=\"http://services/businessdomain/distributionmanagement/maintenance/maintenancerequestsresponses\">", "<ClruInsert>");
        currentEnvelope = currentEnvelope.replace("<BusinessTypeCd xmlns=\"\" xmlns:ns22=\"http://services/businessdomain/distributionmanagement/maintenance/maintenancerequestsresponses\"", "<BusinessTypeCd");

        return currentEnvelope;     
    }

    private class CachedStream extends CachedOutputStream {
        public CachedStream() {
            super();
        }
    }
}

It worked like a charm! :)