I've just had to delete 280 spam comments, going back only a week or so. MovableType has an excellent junk comment filter - 280 comments got caught and marked as junk, and I only got notified by e-mail of 10 comments that were spam. Still - I have to glance down the junk comment list to make sure I've not deleted a genuine comment.
So, hate to do it, but I've had to go to authenticated-only commenting.
Movable Type (of which this blog is an instance) uses TypeKey for this. All you do is create a TypeKey account. You then sign in and have the option to stay signed in for a fortnight. You're then allowed to comment again. As a bonus, you only have to type your name once.
Sorry about that - I'll look into introducing some human-only security feature to weed out the spambots, then I should be able to go allow anonymous comments again.