A solution does exist: providers of mturk-like services could disallow such work items and enforce that (inci-meta-dentally they could use mturk itself to crowd-source spam identification on the cheap).

There is additional work for the service provider but it would seem to me that it does align with their self-interest at some level. I don't think Amazon really wants mturk to be associated with providing a spam work force.

I believe one of the things that CrowdFlower explicitly calls out as an advantage over mturk is quality control (although for this particular solution to work all crowd-sourcing providers would have to do it - in this particular case it takes only one bad provider to enable bad behavior.

As to your hopefully hypothetical question: a risk you're running is that Google will pull out your app from the store. I haven't heard a case with Google but I'm pretty sure apps were pulled from Apple's App Store for manipulating ratings, so the downside could be big (your hard work could amount to nothing).