Renaming the wp-comments-post.php file had a drastic effect on the comment spam – it appears very many blog comment spammers go directly to this file to submit their spam.
After changing the name of this file, not only did the amount of spam fall off significantly but the number of 404’s for this file ballooned – mostly from ip addresses in Brazil or Bulgaria.
Still one or two were getting through. On the offchance that this would increase again, I installed Gudfly’s Authimage. This is a WordPress plug-in which displays an image with some random text that the commenter has to enter in order for their comment to be submitted successfully.
I installed that plug-in this morning and with help on the design side from FrankP, I re-designed the comments page accordingly.
I am now looking forward to significantly reduced comment spam.