"Retarding frivolous sign-ups" is among the worst things you could do. It's extremely difficult to get people to donate crypto to this forum, imagine how much harder it'd be for a forum nobody uses.So let me ask a more specific question: is it viable that you could have reasonable anonymity in a model where a small crypto payment is made to join? If so, that solves our captcha issue entirely and also goes a long way to retard frivolous sign-ups.
A much better model is to be allowed in if at least one other user vouches for you. It's then left up to the individual users what criteria to use:
- CAPTCHA
- Email verification
- Phone verification
- I know them IRL
- Account on a different forum
- etc
There is a much simpler solution: ban anyone who posts CP, ban the user who vouched for them, and don't store the media posted by banned users.Separately, I put my words into action and did some preliminary research into how we could handle the childporn et al. issues. So there are services that let you submit media / hashes of media and get a response back on if it's flagged as child porn.
I strongly encourage you to check out FMS - it solved this problem more than 10 years ago.