Use Google's reCAPTCHA
Step 1: Register your site with Google reCAPTCHA
Go to https://www.google.com/recaptcha/admin and login with your Google Account.
Step 2: Set up reCAPTCHA for your site
WARNING: Changing the CAPTCHA option without configuring it will cause severe issues with it everywhere it is used.
1. Select reCAPTCHA from the dropdown.
2. Click the Edit button.
3. Enter your Site and Secret keys.
4. Click the Save Keys button.
5. Click the Save Settings button.
Test reCAPTCHA on the Storefront
You can test that reCAPTCHA is working by going to your site's login page (www.yourdomain.com/login.aspx) and seeing the option on the "New Users" section.