Like Zomby, I'm trying to deal with comment spammers making a nuisance around here.
I'm afraid I'm going to have to disable comments on older posts. I regret doing that since some people come here via searches and leave interesting comments on posts from months or even years ago. Hopefully someone will come up with a better solution.
MT-Blacklist!
http://www.jayallen.org/comment_spam
And the Turing test plug-in, which should prevent all spambots from being able to post.
Read this, particularly #2 of "Recommended measures".
And if you're not using MT-Close2 to close those old comments, well, there you go.
I tried installing the Turing test (Seng's Scode plugin) a couple weeks ago but was not bright enough to get it to work so I fell back to MT-Blacklist and have been very happy with the results. Installation was easy.
I still see 1-2 spams a day that I have to deal with but the Blacklist stats show that it has been very busy with stuff I never have to see.
I'm running MT 3.12...the upgrade was easy and provides a lot of improved stuff.