How can I add an unremovable prefix to an HTML input field?

I'm not sure about jQuery, but in plain JS, you can try to detect when the field changes and if the change somehow ruined the prefix, add it back (not 100% foolproof - if you want that, you have to just put the "http://" in a span outside the input field and add later programmatically):

<script>
function checkHttp(input) {
    // See if someone deleted 1 character from needed prefix.
    // We can't simply check if the string starts with the prefix
    // since the user may have backspaced out 1 character.
    input.value = input.value.replace(/^http:\/\//, "");
    input.value = input.value.replace(/^http:\//, "");            
    input.value = input.value.replace(/^http\/\//, "");            
    input.value = input.value.replace(/^htt:\/\//, "");            
    input.value = input.value.replace(/^htp:\/\//, "");            
    input.value = input.value.replace(/^ttp:\/\//, "");
    // This doesn't work if someone deletes - say via cut - >1 character.
    input.value = input.value.replace(/^/, "http://");
}
</script>
<input type='text' name='x' id='x' value='http://' onChange='checkHttp(this)'>

Please note that such a weird behavior is really confusing to the user without either explicit explanation beforehand, OR a pop-up (may be non-modal one) explaining that you fixed his mistake for him.