1. Don't grant moderator's privileges to users which you can't trust.
2. Try the following code, adding it to bb_plugins.php (at the end of file):
if($isMod==1 and ($action=='prefs' or $action=='editprefs') and (isset($_GET['adminUser']) or isset($_POST['adminUser']))){
$errorMSG=$l_forbidden; $correctErr=$backErrorLink;
$title=$title.$l_forbidden; $loginError=1;
echo load_header();
echo ParseTpl(makeUp('main_warning'));
//Loading footer
$endtime=get_microtime();
$totaltime=sprintf ("%01.3f", ($endtime-$starttime));
if(isset($includeFooter) and $includeFooter!='') include($includeFooter); else echo ParseTpl(makeUp('main_footer'));
exit;
}