You can create a RegExp from filterstrings first

var filterstrings = ['firststring','secondstring','thirdstring'];
var regex = new RegExp( filterstrings.join( "|" ), "i");

then test if the passedinstring is there

var isAvailable = regex.test( passedinstring );

Here is a test implementation:

// Take care to escape these strings!
// If they are user-input or may contain special regex characters (such as "$" or "."), they have to be escaped.
var filterStrings = ['one','two','three'];
var regex = new RegExp(filterStrings.join( "|" ), "i");

var sentences = [
  "one", // true
  "OnE", // true
  "a rabbit", // false
  "and then two rabbits", // true
  "standing", // false
  "prone", // true
  "AoNeAtWo", // true
];
sentences.forEach(sentence => {
  console.log(
    regex.test(sentence) ? ' match:' : ' not a match:',
    "'" + sentence + "'",
    );
});

Add .toUpperCase() after referrer. This method turns the string into an upper case string. Then, use .indexOf() using RAL instead of Ral.

if (referrer.toUpperCase().indexOf("RAL") === -1) { 

The same can also be achieved using a Regular Expression (especially useful when you want to test against dynamic patterns):

if (!/Ral/i.test(referrer)) {
   //    ^i = Ignore case flag for RegExp