Spamwall

A spambot has now targeted this blog.  (Fake Gucci bags, anyone?)  I’m spending an inordinate amount of time deleting spam.

But the spambot is only hitting one topic.  Does anyone know how to turn off comments on a single topic in WordPress?

I can turn off all comments.  I can turn off comments on pages, but permit comments on the blog.  I can permit comments on pages, but turn off comments on blogs.  I can turn off comments on all future blog posts.

But there seems to be no way to turn off comments on just a single topic.  Does anyone have an answer?