Need help with obscene spam

In the last 2 weeks I've been getting lots of obscene comments posted to my site (lettersfromnyc) and I've had to delete them all manually. This morning I was lucky I only had 10, but that's because I keep checking/deleting frequently and only keep comments open for my 4 most recent posts. The spammers all use unique IP addresses. All this typing has caused my carpal tunnel syndrome to flare up again.

I've checked MT's help feature to see what to do but other than "sanitizing defaults" and adjusting the "Comments template" there's not much. At Munuviana archives there's mention of a plug-in back in Nov '03 but that was prior to MT blacklist being added to the current MT version we have (if I'm not mistaken).

I'd appreciate it if anyone could provide me with some specific solution on how to permanently block these commenters/comments. Thanks.

UPDATE: thanks Rachel Ann, it was set up but I just didn't know that link at the bottom of the comment was how you de-spammed!