The forums used to have problems with automated spam-bots signing up, but since the sign-up mechanism is tied into the Joomla front end and it has a Captcha thingy (those random letters you have to type in) on creating an account, we've had none. You can't post without being a member of course. If it was a pure phpBB forum they could be more lieky to get through.
A couple of times we've had people physcially sign up only to spam about stuff, but this has been rare, and we were able to identify them quickly and either remove the posts and/or delete the offending account. We get an e-mail notifying us when a person signs up, and sometimes those details can be a give away.
So in short, most of it is prevented by making it difficult for automated processes to create an account and login and spam the forum. We haven't had to think about that for ages, luckily.
