Nah, not your fault if some spammers get in. I found this site http://www.stopforumspam.com/
Which has a list of IP addresses of forum spammers, it is a well done site, and it has an API here http://www.stopforumspam.com/apis
that would let you query an IP address to see if it's on their blacklist. I haven't ever set up a forum, but I'm assuming you could add an API call to their list to the registration process on this board preventing spammers by IP blacklist.
But then again blacklisting is only as good as the list, you wouldn't want to block any legitimate users either.
Just an idea, not sure what stuff you have set up anyway.