On Ryan's suggestion I am trying out MT-Blacklist to filter the spam. Basically it takes blacklist I entered and compares any URL's in the ping or comment to the blacklist. Any that match are discarded. You can see my blacklist. It is the same as the master blacklist, except for one entry that was added by scanning my existing spam laden comments. It seems this should work pretty well, at least against the ones I had been getting.