[Mystis]

Sure, just change

$query = $DB_site->query("SELECT forum.forumid as forumid, forum.* as foruminfo, COUNT(post.postid) as numposts FROM forum, thread, post WHERE forum.forumid=thread.forumid AND thread.threadid=post.threadid AND post.userid='".$bbuserinfo[userid]."' GROUP BY forumid ORDER BY numposts DESC LIMIT 0,".$max_forums);

to

$query = $DB_site->query("SELECT forum.forumid as forumid, forum.* as foruminfo, COUNT(post.postid) as numposts FROM forum, thread, post WHERE forum.forumid=thread.forumid AND thread.threadid=post.threadid AND post.userid='".$bbuserinfo[userid]."' AND forum.forumid!=9999 GROUP BY forumid ORDER BY numposts DESC LIMIT 0,".$max_forums);

Replace 9999 with the forumid of the forum you want excluded.