And so it now looks like I've solved the comment spam problem.

I did it without figuring out comment registration in MT3.0. Partly because there's almost no online documentation on the subject, and partly because as Andy pointed out, when faced with having to register, just to be allowed to comment on a website, a lot of people won't bother, and will just move on.

So, now that I'm not spending all my time deleting those thousands of spam comments, I might actually right some more!