Banning TLDs and not just domain names?

I’ve had an ongoing and terrible rash of spam registrations using Drupal 7. I have a question captcha that we rotate but these idiots keep creating accounts. They are using way too many proxies to make IP banning helpful. So I banned the domains they used to register and also banned some free email accounts. So they use throwaway domains or hacked sites. While there is an awesome plugin that allows you to not allow email addresses from certain domains to register (such as all those free email accounts in Russia, China, etc.) I don’t know of any that allows you to band the TLD and not the entire domain name so that everything from .ru or .cn or .online can be blocked. I’d love to add this onto the Domain Registration module but it doesn’t appear to be an option. Has anyone heard of something like this or a hack or module which could help ban these completely useless TLDs such as .online, .website, etc?

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