Rather than stopping bots by having people identify themselves, we can stop the bots by making it difficult for them to make a successful post, or by having them inadvertently identify themselves as bots. This removes the burden from people, and leaves the comment form free of visible anti-spam measures.
Originally from the blog of Ned Batchelder (which I must hasten to mention, boxofmeat gave 100% attribution to), which is decent reading even for 2007, and even if spam seems to have won the day.
While there, I noticed a comment describing something called “Negative CAPTCHA” by Damien Katz. I had to click on through, and was rewarded for my effort with one of the more amusing, responsive and long running comment threads in awhile.
If you have the time… Negative CAPTCHA.
By the way, I have no idea if any of this would work or not anymore. But I really like some of the hunt-and-seek of spam traps and detection, as well as the use of probability theory and such. Naive Bayes can be SO gullible sometimes.