I didnt understand why you dont want to change their usergroup(you could create a replica usergroup anyway) but here is the hack:
Edit newthread.php and newreply.php, find:
PHP Code:
// auto bypass queueing for admins/mods
Before that add:
PHP Code:
if ($bbuserinfo[userid]==X) {$visible=0;}
Replace X with the userid of the user..
If you want to moderate more than 1 user, the code is:
PHP Code:
if ($bbuserinfo[userid]==X OR $bbuserinfo[userid]==Y) {$visible=0;}
Enjoy..