I think the captcha would be kind of useless, but the paywall might be able to prevent any issues in itself. Unless AI start being offered credit cards in the future, it's unlikely it'll be able to pay to get past one. So what's left? Trick some poor human to let it in?

On the other hand, a real life robot has a fairly easy to use off switch. It's called 'destroy the robot with copious amounts of firepower'.