[Lynne]

You could modify the subscriptions.php file directly and not allow them to subscribe. Go to the addsubscription area and add in:

if ( is_member_of($vbulletin->userinfo, x,y,z) )
{
    print_no_permission();
}

There already is an area for no permission, so maybe add it in there:

if (!($forumperms & $vbulletin->bf_ugp_forumpermissions['canview']) OR is_member_of($vbulletin->userinfo, x,y,z))
    {
        print_no_permission();
    }

x,y,z are the usergroup ids.

(Warning: I have NOT tested this!)