User registration supports google reCaptcha feature. reCaptcha is a free service that protects your site from spam and abuse. It uses advanced risk analysis techniques to tell humans and bots apart.
To add google reCaptcha to your form, go to your form and enable reCaptcha support.
And add your reCaptcha Site Key and Secret Key in User Registration -> Settings -> Integraion.
Get your Site key and Secret key from here: https://www.google.com/recaptcha