BTW I strongly recommend to make use of the new de-spamming feature in MTB 2.x:
Instead of just deleting a spam comment on your blog, rather click on the comment’s de-spam link (displayed at the bottom of the comment in the admin interface) or on the according de-spam link in your comment notification e-mail message. The latter is probably the most convenient way to report a spam comment to the master blacklist. See Neil’s World – MT-Blacklist 2.0 RC6 Screenshots for some more information.
Spam is a distributed problem. Fighting spam is a community effort.