Ben Hammersley's Book of Blog this week talks about comment spam in weblogs. He points to two Moveable Type plug-in solutions. James Seng's MT Captcha
The idea is pretty simple: Display an image with a Security Code and demand the user to enter a Security Code manually before allowing posting to go through.
Jay Allen's MT-Blacklist allows for the creation of spam blacklists and allows for the sharing of these blacklists.