Yes, boredom can be a cause for vices, but horses need roughage in their diets. Rather than try to eliminate the behavior, encourage it; But in a more appropriate way. After all, we can’t have the fence or barn chewed down.
My solution was to put 18″ pieces of pine 2×4 in stalls and the pasture. It worked like a charm! The pine is harmless and gives the horse the satisfaction of chewing!