I think saying that a horse only cribs if he is stressed or unhappy is a bit of a simplification. It very well may start out that way, but it is an addictive behavior. There are a lot of things about cribbing that people don't agree on (largely because research hasn't been able to definitively point to a cause or cure for cribbing) but most will agree that a horse who has cribbed for a long time will most likely never stop cribbing entirely, no matter how "naturally" the horse is managed or how happy we perceive him to be.
One thing to consider is that if you can't stop the cribbing, you can at least direct it towards surfaces that are OK to crib on. For example, my horse broke his corner feeder twice; each time I replaced it and the last time I asked for it to be mounted lower (around knee level) and he no longer cribs on it. (And as a side note, I've seen many other non-cribbers in the barn break their feeders, too. Damage to the stalls is not something that's limited to cribbers.) He could crib on it if he wanted to- I've seen him crib on the log in the paddock (used to be part of a cross country course) and it's about the same height or lower than the feeder. But given the choice, he'd rather crib on something higher. He now cribs on a wooden board by his waterer, which is much sturdier. Completely crib-proofing his stall, while possible, would take quite a bit more work, and would most likely mean that the front of his stall would have to be paneled with small mesh- which would prevent him from touching noses with his neighbor. While that would be successful in stopping him from cribbing in his stall, he'd miss out on social interaction in his stall, and he'd likely crib more in turn out (also making him miss out on socialization). I don't think it's worth it to try and stop him completely.