We do not have our own product for eliminating registration spam. But i tested so many plugin for this and i found one. You can install this plugin for eliminating registration spam “Cartpauj Register Captcha”. Hope it will help you.
I am really sorry for the misunderstanding. Required option is not available for recaptcha. Can you please check your public key and private key?. Make sure that you have entered both key correctly. Please check and let me know.