Barking is a dog's way of communicating and often we know its cause, but sometimes it's difficult to determine. So, in order to get your dog to stop barking, you first need to determine the type of barking and its meaning.
If you tried everything that you could do in order to teach your dog not to bark excessively and nothing worked, then you should talk to a professional in this area, a dog behaviorist (behaviour consultant).
The reason why other methods might not have worked is because the problem may be a serious one, like severe separation anxiety, so only a professional in this area can give you the right answers after meeting your dog.
Here you can read more about dog behaviorists and how to find the right one.
If nothing else worked, then some health condition may be the reason why your dog can't stop barking. Take him to the vet to make sure that your dog doesn't have some health issue that's causing him to bark excessively.
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