According to The Chronicle of Higher Education, Google is no longer accepting AdWords ads from mills:
Academic paper-writing services, or “paper mills,” will no longer be able to buy search terms in the Google AdWords program, and thus their ads will no longer pop up in the “sponsored links” sections of a Google search-results page.
You can read the article here, if you’re a subscriber, anyway. They know they’ll have to hand-check sites, but they do seem to have an idea of what a mill is, and does, and how they work, which means they might even spot the more clever ones.
If I see a Google press release, or a more public article about Google’s policy, I’ll link it here.