A popular way for comment spammers to get their job done is to automatically harvest comment forms and forward the data to zombie computers who do the spamming. If you're getting a lot of spam comments to the same blog entries, you're probably a victim of the method.
This Movable Type plugin makes it nearly impossible for zombie nodes to post comments to entries harvested by another computer. Extra hidden values are added to all comment forms, one of those values is a server-computed key that's different for each entry and user, and cannot be used by a different computer or faked.
For the active spam fighter, the plugin also adds "whois" and "report" links to the junk comment editing form in Movable Type's back-end, allowing easy reporting of non-harvester spam. Many providers are more than willing to shut down abusive users. Also, a message like "All spam will be reported" near your comment forms may discourage some human spammers with half a brain.
Movable Type versions: 4.3