The furnace switch is generally located in one of two places, in the utility room near the furnace or in the stairwell to the basement. I suggest you go to a local hardware store and buy the red plate cover with the word “emergency” written on it. This should be enough to prevent accidental use of this switch. I have seen some people place a piece of tape over the switch and that can work as well. I would think once everyone in your home is shown what this switch is for it should be easy to leave it alone. We have technology today that will call us when the house cools down too much, this type of protection is a good idea as well.