[Logician]

If you dont want to use usergroups and access permissions and if you need to hack that's the line you need :

if ($bbuserinfo[usergroupid]!=X AND $forumid=Y) { show_nopermission();exit;}

This allows only usergroup X to pass the line when forumid is Y (You can add it to newthread.php for new threads, newreply.php for new replies)

for specific users choose this one:

if ($bbuserinfo[userid]!=X AND $forumid=Y) { show_nopermission();exit;}

Same applies, only userid X can pass the line in forum Y.

Hope this helps..