Have you tried it?
I've set it up to put a wringer on traffic. I calculated yesterday that spam and attacks on my site have increased 3657% percent versus last year. I don't have any problem denying the attacks, nothing ever gets posted. But it really sucks the bandwidth, even when using dynamic block lists, honey pots, etc.
(http://www.spambotsecurity.com/zbblock.php)
Apparently there is another unrelated project with a similar name for protecting Counter Strike servers. Interesting coincidence given that was a recent forum topic (http://zblock.mgamez.eu/).