This has got to exist already. Today, I was battling trackback spammers. I have MT-Blacklist installed, so they’re mostly just a nuisance, because I have to delete and rebuild all of those entries that they hit.
Then it occurred to me: all of the trackback spam that I have seen has been repeated multiple times on the blog. They never hit just one post, and they’ll often hit dozens or maybe even hundreds.
So, at least for now, a simple solution would be to compare the received trackback with the last N trackbacks to see if there are URLs in common. If so, consider them spam and waste them. I don’t think there’s any legitimate reason to receive the same trackback repeatedly.
Of course, this won’t last forever, because if everyone does this the trackback spammers will just make every URL unique. You can’t work stricly off of the domain provided because of sites like blogspot and typepad. It is conceivable that a popular blog will get more than one trackback from one of these sites within a short period of time.
I would consider implementing this myself, but I’m going to change blog software soon. I’m leaning toward WordPress right now, because it looks pretty slick.