The most useful spam fighting plugins are reCAPTCHA and Bad Behavior. The reCAPTCHA plugin check the commenter is a human or spam bot and Bad behavior makes a list of known spammers and block them from commenting on or accessing your site.
1. Stopping spam with reCAPTCHA
CAPTCHA actually stands for Completely Automated Public Turing test to tell Computers and Humans Apart. The aim of reCAPTCHA is to make a CAPTCHA that can be read by most humans but is too difficult for OCR (Optical Character Recognition) bots to correctly interpret.
1. Sign up for an account at recaptcha.net and add your blog they will provide you two free keys for your blog. This is a global key. It will work across all domains. Make a note of the two keys, as you will need them later.
3. Extract, Upload and activate reCAPTCHA plugin from Site admin.
4. Go to reCAPTCHA setting and add your Public Key and Private Key because without entering these keys it will not activated properly.
5. According to your requirement Enable reCAPTCHA for comments, Hide reCAPTCHA for registered users who can Publish Posts, and Enable reCAPTCHA for registrations.