Google Provides Some AdSense Filtering Tips

Due to an increased show of interest from AdSense publishers in filtering ads, Arlene Lee of AdSense Publisher Support Competitive Ad Filter allows AdSense publishers to add as many as 200 URLs to block. The main reason that publishers would want to do this is to block ads from competitors' sites that may otherwise be served due to contextual implications. To use this feature Google provides the following steps:

Log in to your account at Ad Review Center, which allows users to review ad groups and advertisers before they appear on their pages, giving them added control over what ads are served. From here, users can filter them based on the following options that comments on the Inside AdSense post are a pretty clear indication of this.  Political agendas aside, it should still be useful for publishers to know that these tools are at their disposal.