There seems to be some a bit of it, but it's rather easy for an admin to get rid of. It seems like most of it is on the General Discussion area. Do you guys have moderators? Perhaps, there isn't time to get rid of spam. Anyhow, another problem with forums seems to be profile spam, but it doesn't really affect the look of the forum, though, it might hurt the SEO. Note, a lot of people might just be joining for a profile link.