Unfortunately, the SPAMmers have won this round. I’ve had to disable trackback entirely. Although I had already subverted trackback SPAM by automatically sending all trackbacks to my moderation queue, rather than having them appear on my site, the flood of traffic was a constant attack on my servers, making my site completely unusable. The only solution was to turn off the feature entirely. I will consider replacing it with some kind of Technorati plugin. I’m sure that, in time, the WordPress community will come up with a suitable workaround — but till then I don’t have much of a choice.
UPDATE: This might be a solution. When I have time to spare I’ll try adding it and then turning Trackbacks back on..