The trouble with any sort of captcha or test, is that it teaches the bots how to pass the test. Every time they fail, or guess correctly, that’s a data-point for their own learning. By developing AI in the first place we’ve already ruined every hope we have of creating any kind of test to find them.

I used to moderate a fairly large forum that had a few thousand sign-ups every day. Every day, me and the team of mods would go through the new sign-ups, manually checking usernames and email addresses. The ones that were bots were usually really easy to spot. There would be sequences of names, both in the usernames and email addresses used, for example ChristineHarris913, ChristineHarris914, ChristineHarris915 etc. Another good tell was mixed-up ethnicities in the names: e.g ChristineHuang or ChinLaoHussain. 99% of them were from either China, India or Russia (they mostly don’t seem to use VPNs, I guess they don’t want to pay for them). We would just ban them all en-masse. Each account banned would get an automated email to say so. Legitimate people would of course reply to that email to complain, but in the two years I was a mod there, only a tiny handful ever did, and we would simply apologise and let them back in. A few bots slipped through the net but rarely more than 1 or 2 a day; those we banned as soon as they made their first spam post, but we caught most of them before that.

So, I think the key is a combination of the No-Captcha, which analyses your activity on the sign-up page, combined with an analysis of the chosen username and email address, and an IP check. But don’t use it to stop the sign-up, let them in and then use it to decide whether or not to ban them.