Using Google reCAPTCHA
Google reCAPTCHA is available help protect your site's forms against spam and abuse.
When enabled, a reCAPTCHA logo badge will be added to any page of your site where your form loads. On those pages, reCAPTCHA attempts to determine whether the visitor filling out your form is a real person or a spam bot. If any suspicious activity is detected, a popup window will display a simple task or "challenge" for the user to complete before they can submit. Most visitors will likely never see this challenge, and can submit their form uninterrupted.
reCAPTCHA uses machine learning to collect and study user behavior information to help fight spam. For more, see Google's reCAPTCHA page.