Reverse captcha: only robots can reprove one of the Euler problems on the fly? Statistically speaking we can round the people who can into the outlier group, right?

That's actually interesting:

Like when games detect aimbots, they don't ban people, but put them in an aimbot bracket, so everyone you play with is a cheater.

Provide a captcha that is essentially harder for a human to solve, but trivial for either a human or an AI, and transparently separate them into two communities.