I give up on trackbacks. The spammers have won this round. It's an interesting concept that has been completely abused. All (99%) the pings I get are spam and most of the rest are just from myself to let visitors know that I have continued thoughts from previous posts.
What turned the corner was my implementation of an additional comment spam filter, which works well on comments, but actually performs worse on trackbacks. Solution: turn off that end of the works.
If I find something that works better, I might turn them back on.