This is a tough one. Horses seem to really enjoy keeping you busy with things you never thought you would have to worry about, like preventing them from eating your barn. Many products are on the market, which claim to stop them from doing this. In actuality, most just reduce it. Constant applications are necessary, and they never quite perform as well as you had hoped they would. They do help, though. Another thing that seems to work is Tabasco sauce, or even a bar of soap rubbed into the endangered area. The most sure-fire way of prevention, however, is to wrap all outside corners with galvanized sheet metal, the thickness of which should be no thinner than 16 gauge. Inch and a half by inch and a half angle works just fine. It doesn’t need to cover the whole area, just the corners where the horses can get a start. The metal should be securely fastened with pan-head wood screws to keep sharp edges down, and the edges of the metal itself should be gone over once or twice with a fine file to take off the factory edge. 

If a horse decides to rake his teeth across the middle of a wall, then obviously there is no way to cover that with metal. The best thing probably is to religiously treat the area with the anti-chew treatment of your choice, and tell him to stop it. That usually works. Ha-ha-ha. Actually, a piece of plywood fastened to the wall with wood screws works too. You just move it around when the horse picks a new spot. Not very attractive, but it does work.