One interesting approach I heard about in this domain is the Trolldrossel (German for troll throttle) by Linus Neumann from CCC. He implemented a captcha test for a comment server that would fail with a certain percentage when encountering certain key-words in the comment, even when the captcha was solved correctly. While I have no notes about the effects and the corresponding talk seems to have vanished from the int…
https://linus-neumann.de/2013/05/die-trolldrossel-erkenntnis...