If you don't plan to use this feature at all - an easy way to block it would be to edit the /admin/loginas.php script (make a backup first.
After this:
$user = $userob -> authenticate();
$html = new html;
Add this:
$html -> not_right("This feature is currently disabled.",$Cat);
That'll block it. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />