When you say "spam", you really have to be more specific;
Many malicious users will simply spam threads with useless/inaproppriate (spelling!) materials. This can be stopped slightly by enabling "Spam control" of about 30-60 seconds.
One thing you MUST disable is HTML in signatures. Users can create spam codes such as opening multiple windows in their signature, usually linking to more unacceptable sites.
Limiting links to other sites is pretty useless, as the point of forums are to create circles off communities of the same subject, but if you feel some members will leech your community strongly then limiting links may help.
Limiting the number of characters in a signature is a good move, usually to about 300-500 characters in my opinion.
The sad truth is, though, no matter how hard you try "rage" or "grief" groups will always be able to bypass these anti-spam measures, but they usually have a motive i.e. they have placed some sort of attack on the group in a verbal manner, or "bashing" them on their site/forum, or they find the content humorous in one way or another and simply spam it for amusement. The best way to counter-attack these are simply to stay calm, delete and ban as many users as possible (however many use proxies so keeping them away permenantly is impossible in some cases), or if things get too over the top, simply take the boards offline for a week or so.
The thing to remember is that these attacks are rare, and you may come accross the odd immature user, but they are easily dealt with.
Unsolved 
