In Live Content Simply set the CAPTCHA option to "Default" and comments will require a CAPTCHA verification.
reCAPTCHA
Using Google's reCAPTCHA is just as easy. Set the CAPTCHA option to "reCAPTCHA", select a theme, and provide your public and private key.
You can generate your keys by signing up for free at http://www.google.com/recaptcha
From Manage -> Settings -> Comments
