string username = Regex.Replace(_username, @"(\s+|@|&|'|\(|\)|<|>|#)", "");

I believe, best is to use a regular expression here as below

s/[*'",_&#^@]/ /g

You can use Regex class for this purpose

Regex reg = new Regex("[*'\",_&#^@]");
str1 = reg.Replace(str1, string.Empty);

Regex reg1 = new Regex("[ ]");
str1 = reg.Replace(str1, "-");

Use \B[!-/;-@]+\s*\b:

var result = Regex.Replace(s, @"\B[!-/;-@]+\s*\b", "");

See the regex demo

Details

• \B - the position other than a word boundary (there must be start of string or a non-word char immediately to the left of the current position)
• [!-/;-@]+ - 1 or more ASCII punctuation
• \s* - 0+ whitespace chars
• \b - a word boundary, there must be a letter/digit/underscore immediately to the right of the current location.

If you plan to remove all punctuation and symbols, use

var result = Regex.Replace(s, @"\B[\p{P}\p{S}]+\s*\b", "");

See another regex demo.

Note that \p{P} matches any punctuation symbols and \p{S} matches any symbols.

That depends on what you mean. If you just want to get rid of them, do this:
(Update: Apparently you want to keep digits as well, use the second lines in that case)

String alphaOnly = input.replaceAll("[^a-zA-Z]+","");
String alphaAndDigits = input.replaceAll("[^a-zA-Z0-9]+","");

or the equivalent:

String alphaOnly = input.replaceAll("[^\\p{Alpha}]+","");
String alphaAndDigits = input.replaceAll("[^\\p{Alpha}\\p{Digit}]+","");

(All of these can be significantly improved by precompiling the regex pattern and storing it in a constant)

Or, with Guava:

private static final CharMatcher ALNUM =
  CharMatcher.inRange('a', 'z').or(CharMatcher.inRange('A', 'Z'))
  .or(CharMatcher.inRange('0', '9')).precomputed();
// ...
String alphaAndDigits = ALNUM.retainFrom(input);

But if you want to turn accented characters into something sensible that's still ascii, look at these questions:

• Converting Java String to ASCII
• Java change áéőűú to aeouu
• ń ǹ ň ñ ṅ ņ ṇ ṋ ṉ ̈ ɲ ƞ ᶇ ɳ ȵ --> n or Remove diacritical marks from unicode chars