The NoLiP plugin enables you to add the “nofollow” attribute into links in a post within a specified category. This plugin is useful if you are doing sponsored posts. You can set the plugin to add the “nofollow” attribute for posts older than “X” days.
Originally the Nofollow Links in Post Plugin found here:
http://www.themiak.com/wp-plugins/nofollow-links-in-posts/