The $totalthreads and $totalposts is totalled up around line 620 of the forum.php file. So, it's probably easier to just not add in the $totalthreads and $totalposts from that forum. Put a condition around the lines like this:
PHP Code:
if ($forum['forumid'] != xx) {
$totalthreads += $forum['threadcount'];
$totalposts += $forum['replycount'];
}
Change xx to the forum you don't want to include in the thread/post count.