Spam - How to block special emails?

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Since two weeks I get tens of daily spammer registrations. The old ReCaptcha is enabled but does not help any more.

I see the registrations with the email from the special domain, how to block this domain?

I see only a field for allowed email domain but not for disallowed.

I think askbot should implement the more modern ReCaptcha from Google, see here google.com/recaptcha/intro/