They will fall down through a hole and die.
How to make a falling floor in minecraft.
First and probably the simplest set up a room that is 2 wide with a trapped chest at the end a trapped chest will give off a redstone output when opened so hook it up to a redstone torch to invert the signal and then put sticky pistons that will retract the floor.
Above ground version here.
You can also put lava or spider webs.
Insert random nonsense here.