Add delay after invalid user registration.

[Steve1951]

My server routinely gets hammered from IP addresses outside of the US attempting to register new users (up to 3 or 4 attempts per second) per IP address. So far none have been registered because I have good human verification policies. Is there a way to add a delay (15 to 30 seconds) before a specific visitor can attempt another registration attempt? Any suggestions will be appreciated. I'm running version 4.1.11.

Thanks,
Steve