it is better to put onsubmit attribute for form element and true/false return from confirm will allow submission or block it. Something like this should work:
<form method="post" action="" onsubmit="return confirm('Are you sure you want to delete this account?');">
<input type="hidden" name="usr" value="<?php echo $user; ?>"/>
<input type="submit" name="sus" value="Terminate User" />
you can make onsubmit call a separate function of course, as you had before if you need to put more code in there. But this gives you an idea.