Also, other blog has a hidden field, that should be empty, but bots like filling all fields.
I'd try those stupid tricks, and if they fail I'd try to put Spambayes as a filter. It was nice because it has good/bad/unusual, and you may like to take a look at unusual stuff to detect false positives. (I'm not sure if there is a better alternative to Spambayes. I used it like 20 years ago.)