I also run a couple of forums and behind the scenes you have a number of options to administer things. Usually in the form of tick boxes, eg "Allow members to use signature yes(tick)/no(no tick)" etc.
If you simply allow people to register and post then you get spammed to death cos its all automatic and they can register lots at a time. I employ a 'captcha' which weeds out quite a few of them but still a number get through
So like Nick I had to employ registrations being approved which is a pain but I get an email every time someone registers and I can usually tell by the username that its a spammer. 99% of them are random names that don't resemble anything in English. As long as thats the case I just ignore them until I get one that might be a real person (sometimes it is, and sometimes the spammers get lucky) so I then go into admin mode and check through the names awaiting registration same way as Nick, check the IP addresses, once I've dealt with any real folk, the rest get bulk removed. It takes me a few minutes every few days, but I ain't had a spammer actually post for quite some time now.
My only minor worry is that someone real will register with a really silly name and get removed lol
