I do like the Ancient Alien theory that people were genetically engineered to be slaves to Space Alien Overlords but quickly turned the tables on them and so they left. That would make us the assh*le species of the Universe, and that appeals to me on so many levels.
Any reasonable attempt to genetically engineer a population for enslaved labor would result in a kill switch being implemented in said genetic code.
So, if this is the case--say they are doing this now, there's nothing we may be able to do once they decide to put us to work. I wager that a civilization intelligent and crafty enough to pull this off would embed a switch in our genome that could not only kill, but potentially work to override our free will in our nervous system, placing us in a catatonic acquiescent trance.
The brain is a series of synapses, after all, and if they could engineer a protein(that could pass the blood-brain barrier) shaped to interfere with the right junctions, assuming they did their research, which I'm sure they would, I don't see this as implausible. Assuming the brain would not short-circuit and go haywire.