Most of them do that now - the better markets have gateways that are CAPTCHA'd and spread out over other providers that then proxy to the real hidden service (you can do this with a second step over Tor again)
Checkout this project which is now being more widely deployed to prevent a lot of these attacks:
Checkout this project which is now being more widely deployed to prevent a lot of these attacks:
https://github.com/onionltd/EndGame