First, you guys do a good job of getting rid of spam.
However, it does show up on RSS, and is probably a bit of work for you.
It might be easier to add a few options to your forum board, to reduce the incentive for people to spam:
1. Turn on rel="nofollow" on hyperlinks. This will tell search engines to not follow links on the board (you can add a whitelist), so spammers have less incentive to spam:
2. you could also moderate new users, and use captcha questions: