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).
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).