How to disable Ctrl+U using Javascript

Your problem is the return statement.

Though I would suggest you use addEventListener and the like, this is a working copy of your code:

document.onkeydown = function(e) {
        if (e.ctrlKey && 
            (e.keyCode === 67 || 
             e.keyCode === 86 || 
             e.keyCode === 85 || 
             e.keyCode === 117)) {
            alert('not allowed');
            return false;
        } else {
            return true;
        }
};

This is finally what I got to disable Ctrl+U:

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<script>
document.onkeydown = function(e) {
        if (e.ctrlKey && 
            (e.keyCode === 67 || 
             e.keyCode === 86 || 
             e.keyCode === 85 || 
             e.keyCode === 117)) {
            return false;
        } else {
            return true;
        }
};
$(document).keypress("u",function(e) {
  if(e.ctrlKey)
  {
return false;
}
else
{
return true;
}
});
</script>

Its simple just use the following code it will disable only Ctrl+U while Ctrl+C, Ctrl+V, Ctrl+S etc will works fine:

<script>
document.onkeydown = function(e) {
        if (e.ctrlKey && 
            (e.keyCode === 85 )) {
            return false;
        }
};
</script>