AdSense Publishers who are serious about making big bucks with Google AdSense tend to be aggressive with the managing sites they do not want to show up in their ads. There was even a small unscientific study, where a WebmasterWorld member tested removing his AdSense filters and noticed a downward affect on his revenues. I was skimming Cre8asite Forums and found a thread named Adsense Black List, interesting.... The thread is probably at several other forums, but I found it here first.
It links to a tool at http://www.adsenseblacklist.com/ that helps you generate an AdSense filter for your industry. The tool will "filter low paying google ads which link to MFA (Made for Adsense) sites AND we will provide you with tools that will help you to substantly increase your AdSense revenue, all for free." You can even submit your own list to help the rest of the AdSense publishers.
Ah, but is there any concern for blocking your competitors unfairly on all other sites? Time will tell.
Forum discussion at Cre8asite Forums.
Update to this service under AdSense Black List Revisted.