Answer:
An occasional bark or so is okay, but it is unacceptable if the dog is barking unstoppably.
When the dog starts barking continuously again, squirt a stream of water at the back of its neck from a spray bottle, then ignore the dog for the next couple of minutes. This reinforces the idea that barking is bad - it'll only result in an uncomfortable squirt on the neck. However, bear in mind that this may not affect individual dogs, so please seek professional help from a dog trainer.
If the dog only barks in certain situations, reward it with a food or a toy the next time it stays quiet. This creates the idea that staying quiet in this situation is good as it'll result in a reward.
You may also reduce continuous barking by taking the dog out for longer walks, as this reduces the excessive energy it uses for barking. It is also important that you reinforce your position as a leader in your home. Dogs are social animals that live under a hierarchy system, and leaders can bark as often as they like. There is a chance that your dog considers himself to be the alpha.
Whatever you do, do not use physical violence to punish the dog, as this is extremely cruel and will negatively affect the dog both physically and mentally. It is strongly recommended that you seek professional help from a dog trainer when facing problems in dog training.