[Admin]

Thanks amykhar.

To do it per forum, undo all changes you (might) did to showthread.php.

Now, add what's in red (in showthread.php):

$forum=getforuminfo($thread['forumid']);

if ($bbuserinfo[userid]==0 and ($thread['forumid']!=1 or $thread['forumid']!=2 or $thread['forumid']!=3 or $thread['forumid']!=4)) {
  show_nopermission();
}

Note: appears twice, add in both places.
This will prevent all guests from viewing threads in forums 1, 2, 3 and 4.

LanciaStratos, undo all changes.
Add what's in red:

$forum=getforuminfo($thread['forumid']);

if ($bbuserinfo[userid]==0) {
  show_nopermission();
}

Same note, do it twice.
(no difference, just "nicer")