I wouldn't euthanize one. I would do all the tricks I could manage to maintain them. The mare that I mentioned in an earlier post is a well bred WP mare last appraised for $18,000 and her last foal sold for $9,000 as a yearling.
Some horses are more extreme than others. If you don't own the horse, then you will probably be more frustrated than if it is your horse. As a barn owner you may need to place lots of restrictions on the horse to save your property, sanity and not let the habit spread.
AKA - 24/7/365 cribbing collar, owner MUST have SOMEONE/ANYONE work the horse x number of days a week (might be a little side income for BO if they were to offer for x $$), owner is responsible for immediate repair of property, horse must stay in hot wire enclosure with flat low water trough and ground feeder only, etc...etc....
As a property owner you will need to be specific and communicate well. Put it in writing also. Wood chewers are actually a bit worse than a lot of cribbers. One that also chews needs also either full hot wire deterrent or all wood covered with metal shields. Boarders should be screened coming in for all theire horses habits - cribbing, chewing, rearing, kickings, etc..... Have a stipulation plan in place for those horses.