I've decided reluctantly to take the advice of PQ, posted in the comments yesterday, to turn on comment moderation for this blog. I don't like to have restrictions on comments; I've been very lucky to avoid trolls and abuse of commenting privileges, with the notable exception of the spam that I've had to put up with. Google apparently just won't take action against spam. I promise to avoid censorship and to promptly approve all legitimate comments.
Normal blogging will resume tomorrow.