If you ever need to disable the tab key on an element with mootools, it is quite simple. You just need to add an event listener on the keydown event and check the event.code property — 9 being the code for tab (check this out for other key codes and everything you could possibly want to know about javascript keyboard events).

The following example will listen for the tab key event on an element with an id of ‘input-element’ and not allow the user to tab forward out of the element. However, if the shift key is pressed (detected with the event.shift property), it will let the user tab backwards out of it.

$('input-element').addEvent('keydown', function(event){
  if (event.code==9 && !event.shift) {
    return false;
  }
});

Note: Consider carefully before disabling the tab key, as it’s generally a bad idea to force users to use the mouse.

Thanks to this post on doing similar things in jquery for pointing me in the right direction..