I’ve started using a Captcha for the comments. Over and above the spam protection I’ve been using, I’ve been deleting more than 10 spam posts per day. This is just a nuisance, but it has become a nuisance that is grating.
I’m pretty sure this captcha is eminently defeatable. You just need to examine the page for the details and do simple arithmetic. I suppose this plugin is going on the theory that you don’t (usually) need to be the most secure; you just need to be more secure than the other guy.
If you can’t perform the requisite arithmetic, you’re not fit to comment (hint: that computer thing you’re using almost certainly has a calculator available).