Google already has a similar mechanism in place where they require captchas from IP addresses that abused Google APIs previously. The combination of that with

Google already has a similar mechanism in place where they require captchas from IP addresses that abused Google APIs previously. The combination of that with Google Shield might actually work to inform the users, but then again users are confronted with similar warnings from abusive ad networks all the time, and probably learned to click them away fast and forget about it.