This article was popular recently and was published everywhere. It explains that there are peolple being paid to click on clients' AdSense ads.
This only works where cpc ads are displayed on small publishing sites because the small publishing site makes the money and gives a cut to the clickers.
One possible solution would be to pull the ads from "partner sites" and only allow the ads to be displayed in the serps and on larger established sites where they would be less likely to engage in paying clickers.