I just can't take it anymore. I'm turning off anonymous commenting completely.
I've tried Akisment, Captcha, Mollom, all of them. Nothing works.
I believe the only way a site can prevent spam altogether is by requiring accountability.
I don't recall the name of it at the moment, but Google was working on some sort of verified web identity tool. Only if a user comes to a site with some sort of valid 'credentials' do I think he should be trusted.