Polygraph? You mean the thing that might tell you that a plant is lying? Even with today's technology, those are not reliable.
Nazi Germany did have tighter control after it came to power. But by that point, they sent out people to beat up members/supporters of other parties and openly voting against them could get you disappeared.
Thus, stoning voters of those 'elections' would be tantamount to 'I am gonna kill you for re-affirming a party that would fake the results anyway because you did to not want to be punished'