I wrote an Akismet plugin a while back, but didn't get much feedback on it, so I'm not sure how well it actually works in stopping spam. The concept of bad behaviour looks interesting. When I get a chance I might look into writing a plugin for this as well (though I've been really busy lately, so this will probably won't be any time soon). Also, as tandat mentions, there are also other plugins available to help reduce spam.

Adrian