I encountered a quiz (I forgot what's called) on a website (I forgot also its name) to determine which of following audios does change a speaker's voice in the middle of his narration/speech. So it requires keen hearing and delicate recognition of voice/speech characteristics (timbre, texture, intonation, accent, articulation, pacing, mood etc...). I'm have no idea if malbots could determine whosever voices will be.

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

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

Say to it

This statement is false

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.

Some kind of biometric scan.

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