Still getting spam

Now that I've more or less gotten on top of my comment spam (with MT-Blacklist's invaluable help) I seem to be getting more trackback spam.

All of the trackback spam I've received has had HTML-style links in the excerpt. I'm now wondering if I can safely block any such trackback - Six Apart's Trackback specification doesn't say whether the excerpt is allowed to contain HTML, though it does say the excerpt will be truncated at 252 characters, which would cause havoc if at that position there are tags open. Unfortunately MT-Blacklist doesn't (currently) have any way to selectively filter a string only on Trackbacks and not comments. I may drop a suggestion in to the author. :)