I'd guess delete them individually...IF you can find where they're tracked back to.

I cannot locate some of my trackback spam....

The email notice tells you which post it is.

Would it be possible to assign very large random numbers as trackback ids? I think that should reduce trackback spam.

You can despam one of them using MT Blacklist, then use Blacklist again to zap all the rest automatically, using the De-Spam button. (You have to change it from searching for comments to trackbakcs.)

Or I could do my daily-ish sweep and clear them all out.

Thanks. The attack seems to have abated (for now) I closed trackbacks on every post that was hit, so that might help. But I'll remember Blacklist for next time - it didn't occur to me that it handled trackbacks, d'oh!