Armour stands can be placed in the void using a piston. Simply just put an armour stand on a piston (of any sort) that is facing upwards and activate the piston roughly 2 or 3 times. You should see the armour stand change its stance on the first activation, then after, sink into the ground. If you check by breaking the blocks around you will see that it may be floating in the void or just simply underground. This works even if there is armour and/or a weapon on the armour stand and could be unfair on some servers for hiding armour.