Show a picture, video, audio clip or text designed to elicit an emotion. Ask how the user feels.

LLMs, IIRC, are really bad at IQ-test type questions that require abstract reasoning, especially if they require multiple steps. So, something like

The box is yellow and red.
If the box is yellow, it is good.
If the box is blue, it is unhappy.
If the box is good and happy, the box is awesome.
If the box is red, it is happy.
Is the box awesome?

is what I'd use.

I'd ask for their cell number and send a verification code. That'll stop 95% of all duplicate accounts. Keep the hash of their phone number in a hash list, rather than the number itself. Don't allow signups from outside whatever region you can SMS for free.

I realize this would mean relying on an external protocol (SMS), but it might just keep the crap out. Would help for ban evasion too, at least within an instance.

Captcha or recaptcha is good enough imo, no point in reinventing the wheel. Alternatively, split instructions in an email and on the website. For ex: Send email with What is the square of 3 (sent as an image for every word) And on the website Email + 25 = xxxxx

Wait a minute - GPT-4 - is that you asking this question?

The trolly problem as captcha. AI's literally cannot answer that.

Some kind of biometric scan.

Any bot? That's just impossible. We're going to have to tie identity back to meatspace somehow eventually.

An existing bot? I don't think I can improve on existing captchas, really. I imagine an LLM will eventually tip their hand, too, like giving an "as an AI" answer or just knowing way too much stuff.