Anti-spam measures

I’ve noticed quite a few spam comments coming through, in spite of the Captchas in comment forms (Captchas are the ones with a picture of some numbers or some text, which you have to type into a box to prove you’re a human). I have now enabled Captchas for logged-in users just to see whether that helps improve the situation. Spam comments can have a bad effect on the performance of the site, apart from anything else.
Hopefully this won’t cause too much inconvenience for legitimate myblog.users in the meantime.