My mom used to show golden retrievers before she had me. The best answer I can give you? Redirect. If they're chewing on something they shouldn't, replace it with something they should, like a chew toy. If they're doing something they shouldn't, redirect them to doing something they should, then praise them for doing the thing they should be doing.
Actually I am still yet to own a dog because I am always afraid of dogs.
But if perhaps I own a dog and I discover a problem in the dog whether mentally or physically, I will try my best to solve the problem either by visiting the vetenary doctor and also by using the prescribe drugs so as to cure the problem in the dog.
But if also I found out that the problem can not be cured, I will kill the dog earlier to avoid further danger. With these I have been able to deal with the problem either by solving the problem and making the dog to live or by rather killing the dog when I discover the problem can not be solve.