Let me join Alex Tabarrok in turning off trackbacks:
Marginal Revolution: Trackbacks are Off: Due to an onslaught of spam, we have stopped accepting trackbacks. If Typepad improves their spam filters we will turn trackbacks back on but at present the ratio of spam to real trackbacks is over 100 to 1. We like trackbacks and regret the inconvenience. Posted by Alex Tabarrok
It is just not possible to police them. I don't have time with comments to keep the spam and the trolls down in any but the most half-a**** fashion. I can't deal with trackback spamming and trolling as well.