Efficient spam bot protection

Wordpress blogging system comes with the "Akismet" learning spam filter for the comments. Unfortunately this filter has proved to be fallible and missed 1-2 spam comments out of every 100.

Almost two weeks ago I had to implement extremely simple spam bot protection in my weblog on top of the Akismet.

That's the picture that I see every day since then:

Akismet filter is still working in case next-gen of spam bots will learn to check every single check box they see.

PS: I'm disappointed that spammers do not simply hook some fast evolutionary algorithms to the spam delivery tool (trained on "per publishing platform" basis). It would've been interesting to fight these.