Perhaps you could fight it by doing statistical analysis on review patterns. If the device is only ever giving great and/or crappy ratings and the same group of devices rates the same group of apps the same way, you could detect this.

Edit: Of course, they could fight detection by having a queue of apps to review and an array of devices. Randomize the matrix and periodically retire a device to prevent it from gaining too much history. Since it's running in a machine it would likely be in pristine condition and capable of being resold as new. Heck, they could buy one, use it for a period of time then return it.