I'm surprised this thread has gone on for as long as it has, when the answer is quite simple, if long-winded in explanation. While it is certainly not okay to beat your pet or hit it repeatedly, firm but reasonable physical discouragement is the most effective way to teach your pet not to do something. This, of course, has to go hand-in-hand with reward based encouragement for good behaviour and obedience, or it just won't work.
What people often don't understand is how to go about discouraging a pet, and the reason for this is that they don't understand how difficult it is for a house pet, even the more intelligent breeds of dog, to make a connection between an event and its result. If your dog savages your mail and you hit him for it hours later, the animal will not understand why you are hitting it. Even if you show him the mail, he won't get it. You need to act quickly, within a few seconds. Give the animal a firm swat on the nose or behind, and say "No". The animal will associate its recent behaviour and the word "No" with its punishment. It will be discouraged from repeating this behaviour, and you can discourage it in the future simply by saying "No", without the need for physical discouragement.
A dog's inability to make connections between punishments and not-so-recent actions is why kennels and pounds will usually tell you not to punish your pet at all if it has behaved destructively in your absence. If you do, even if you take the dog back to the scene of its misbehaviour, the dog will associate the punishment with your return, NOT its behaviour. This can be very damaging to a pet, not to mention your relationship to it.
People who often punish a dog's destructive behaviour too late will notice the dog behaving sheepishly upon their return, such as lying down or hiding its face, and mistakenly believe that the animal "knows it has done something wrong", and will punish it for its disobedience. This is not the case - the animal has begun to associate punishment with its owner's return, and is attempting to show subservience to appease its owner. Punishing an animal in this state can have dreadful effects, and for this reason I would strongly discourage anyone reading this from ever hitting an animal that is behaving meekly. Simply put, it is afraid of you, and that is never a good thing.
With this in mind, you can (and should) gently discourage your dog from bad behaviour with a firm tap on the nose, so long as you act quickly. And don't forget to reward it with treats for good behaviour and obedience, either.