MacKeeper ads pop up constantly. I can't get rid of them. I tried blocking cookies in security and privacy. No luck.
If you don't mind poking under the hood of your system, you can edit your /etc/hosts file to block known advertising domains. I prefer this method to using browser-specific extensions since it works at the system level and does not depend on what browser I am using. For references, see http://lifehacker.com/5817447/how-to-block-unwanted-ads-in-all-applications-and-speed-up-web-browsing-with-the-hosts-file.