MT-Blacklist rules, but if you’re running MT 3.11 (I’m still on the release before that, because 3.11 contained only fixes for bug I’m not seeing), take a look at this site before installing:
By all means, if you run MT, install MT-blacklist. It makes comment spam so much less a problem, and the beauty of it is, it collects blacklisted sites automatically.
I’m a huge fan. I didn’t have nearly the trouble Laura did on her install – the version possibly, or stupid good luck.