Well, preventing the unit from using spells is easy enough, simply cast Silence on it. As for regular orders such as attack/move/etc, it's somewhat trickier. One thing you could try is activating a trigger which spams orders for the unit, like every 0.05 seconds or something like that, so even if the player attempts to give an order to the unit, the trigger will give a new order within 0.05 seconds, so it will seem like the player has no control whatsoever.