Apparently Google are wanting to get rid of ads leading to low-quality pages. Such pages could be full of affiliate links, “squeeze pages” that try to capture email addresses without much content or pages that are intended to “flip clicks” (by buying low-cost clicks from Adwords and flipping them by having high-paying Adsense ads on the landing pages).
Apparently Google don’t say you’re banned, they say your minimum cost per click is $5, $10 or somesuch. LOL. Same thing.
Check out the SE Roundtable posts here and here for more information. Those posts comment on Webmaster World threads, which require a subscription to access but here’s the official information from Google themselves and a long thread at Digital Point if you’re interested in more reading!