To add the captcha verification, the best tool is WP-reCAPTCHA. With this program, you can easily enable the captcha challenges on the comment forms, preventing the hackers and spammers from irritating you daily with spam comments. Let me here tell you that humans and bots can only insert this captcha cannot even recognize the words mentioned in the captcha; thus, you will be assured that your site is protected against useless and spam comments.