From the Hostmatters forums, a bit of wisdom on circumventing nasty MT comment spammers:

The key is that spammers have automated scripts that look for Moveable Type blog sites and they then post to our comments using a direct call to the "mt-comments.cgi" script. If you installed Moveable Type into the default directory (/mt) then they know exactly where the script is and how to call it.

The solution is simple: rename the script to some odd name (ex. qwerty.cgi) and edit your mt.cfg to point to the renamed CGI script. Look for the line that is commented out and reads "# CommentScript mt-comments.cgi". Uncomment the line and change the name of the script to the new name. You need to rebuild the site before it takes effect. Users will not be able to post comments while you are doing this but the entire process only takes a few minutes.

I've been getting slammed with around 30 comment pr0n spams a day lately, trying this method in 3, 2, 1...