Have you any kick flaps on the stable doors? It wasn't something I came across while living in Canada but it is very very common here. Incase you don't know, a kick flap is a "latch" at the bottom of the door, normally made of metal. It swivels around and there is a catcher on the door frame. It is far too low for a horse to reach as it is just above the ground, and is great for added security. They are very cheap and all it takes is a screwdriver to put up.
Stop for a minute, open your mind, learn. You may not agree with what I say, I may not agree with what you say but we will both learn something new.