I just installed the WordPress plugin for Http:BL, which makes it really easy to tap into Project Honey Pot’s ban list to keep the bad bots off this site. You can also easily create your own honey pot and turn in offenders.
I’ve been using Akismet to filter spam comments (thanks for the recommendation, Emily S.!), which is why you don’t see hundreds of ads for drugs and porn in the comments on this site. It catches everything and hasn’t falsely caught any legitimate comments yet. But blocking them here is only defensive — I’m glad to be able to help crack down on them overall.