[Erwin]

I'll tell you how I do it. You can achieve the same result without hacking by doing this:

Open phpinclude template, add to the bottom:

if ($bbuserinfo[usergroupid] == 8) {
   header("HTTP/1.0 404 Not Found");
   header("Status: 404 Not Found");
   exit;
}

Change 8 to the banned usergroupid you use. Done!