A CAPTCHA is a program that can tell whether its user is a human or a computer. You've probably seen them — colorful images with distorted text at the bottom of Web registration forms. CAPTCHAs are used by many websites to prevent abuse from "bots," or automated programs usually written to generate spam.
No computer program can read distorted text as well as humans can, so bots cannot navigate sites protected by CAPTCHAs.' Source: reCAPTCHA.
However, if this did not solve your problem in WordPress, you may want to do the following.
In WordPress dashboard under Settings > Discussion, Default article settings section un-check 'Allow link notifications from other blogs (pingbacks and trackbacks)'. It solved my spam issue. I used to get 800 spam comments per week. By installing reCAPCHA in my blog it reduced to 25 spam messages per week. Then I disabled the pingbacks and trackbacks and now I am getting zero spam. Moderate all your comments, that really helps too.