Multiman, if the problem is only from these ever revolving new affiliates, then the new In Trial/On Hold / Disabled policy would be the solution to that. Google could see the past performance of other affiliates for the same keywords and put them 'In Trial'.
Also I don't believe that it is always a new affiliate cropping up in the space of old affiliates. For some of my keywords, I do see that the same affiliate/ affiliates have been holding ground for a long time. The only way they can hold ground is if the user finds them relevant.