[Opserty]

Banning them from the server/site is your only real option as, as soon as they visit your site their data (IP e.t.c.) is stored automatically in the session table, which is where the guest count comes from. Checking all the guests in this table against the IP Ban List may cause a performance hit especially as your forum grows, if they are permanently banned from the forums I would just suggest snakes1100 method.

I'm not trying to skirt round the issue but the gains from going to the trouble of editing files and extra work done by the server don't seem worth it. If you really want to then you could try editing the query in the index.php file. (You'll need to search for it but I'm sure its commented somewhere). If there is a common IP then you could just count users that do not have that IP I guess.