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 internet, it supposedly worked quite well by forcing any 'obscene' comments through additional rounds of captcha without telling that that was the reason for them to fail, thus demotivating the submitting person to do so.
I think this is a really bad solution that only reinforces bad practices of hiding content. Even worse than direct censorship, it generates more problems than it solves.
When everyone knows it, I agree. As long as it is not mentioned anywhere, and people do not write hundreds of comments, its just a failed captcha that appears more often when someone writes something in range, possibly forcing them to rethink their argument. Its no solution for all, more an interesting thought.
While I have no notes about the effects and the corresponding talk seems to have vanished from the internet, it supposedly worked quite well by forcing any 'obscene' comments through additional rounds of captcha without telling that that was the reason for them to fail, thus demotivating the submitting person to do so.