Use creosote or an anti-crib liquid to make wood less appetizing to your horse. Or mix up your own concoction of cayenne pepper and petroleum jelly. Don't leave racks or wooden feeders that your horse can get hold of. Keep the top of the gate closed and cover the edges of the stall with metal trim. If your horse is cribbing out of boredom, turn him out more frequently or for longer periods to allow him to exercise. You may also want to provide him with toys to occupy him when he's in his stall.
For more information, please read the story Keep Your Horse From Chewing on his Stall.