The Bad Behavior plugin I was talking about didn’t quite do the trick. Since I installed it two days ago, it has stopped about 35 spammings, but about 10-15 got through anyway. In this case I should turn on verbose logging and send a problem report to the author, but I just can’t be bothered.
Instead I looked at some other (complementary) solutions. As it turned out, there has been a spam plugin ready to go in my WordPress plugin directory, I just haven’t activated it. So, I got myself a WordPress API key (required by the plugin) and activated the Akismet plugin.
It immediately recognized 18 comments as spam, so I’d say it looks promising. Let’s see how it does in the future. Hopefully, Bad Behavior and Akismet together will ensure that spammers can no longer bloat my blog (a malicious spin-off from Pimp my ride).