The ad filter tool allows a publisher to specify which advertisers they do not want to show up on their site. It was specifically designed to help you block competitors for displaying on your site, but is now used to block "MFAs." The problem for many publishers is that as time goes on, they run out of room and they need more than 200 lines to add to their block list. MFAs keep appearing and that is just the nature of the business.
Is there a way to increase your 200 maximum limit to more? I am not sure, but it is suggested that you email your AdSense representative to see what they can do on your behalf. Typically, Google can accommodate certain publishers and advertisers.
Forum discussion at DigitalPoint Forums.