Apart of Captcha!, to fight trackback SPAM I also use Trackback Validator, which works very well but has a little problem however: regardless the trackback is ham or spam, you always get it via email if “mail comments” feature is enabled in your WP panel. So you might end up with your mailbox flooded of spam trackbacks (even though they’re correctly filtered in your blog).
Due to this, I’ve made a little hack to fix this. So only valid trackbacks are mailed to you, not the SPAM ones.
Download it here.
Update: (2007-04-01) It seems I’m not the first one to do something like this. Although it uses another approach (hacking WP directly).