If you want to keep out porn spam, what you really want is probably a machine learning solution, e.g. Naive Bayes, of which many free implementations exist.

If you want to let people say "ah, fuck me ;)" but not "I'm going to rip off your head and doody down your throat," you probably need moderation, meta-moderation, etc.

If you just want to keep Teh Naughty off the site, you can probably do what another poster mentioned and have a set of exceptions that you manually add to when you notice particular false positives. In this case, you could probably start with a CPAN module. I don't personally think this last goal makes sense, but it's not my site...