Without proper training a dog that barks excessively will never unlearn this behavior. However, as you're training your dog, there are a few tools you can use that may interrupt its barking quickly. The key is to get the dog to associate its barking with something that is mildly disconcerting — in this example, a loud noise that does not harm the dog but will get its attention and pause its behavior.
Put some loose change in an empty coffee can. Don't fill the can too much — you want the rattling change to be as loud as possible.
When your dog starts to bark, shake the can in front of the dog. The dog will stop barking immediately because of the sound.
Do this every time your dog barks a lot without good reason. Soon your dog will decrease its barking because it hates the sound of the change in the can.
Issue a command immediately after shaking the can. It can be "No," "No bark," or "Hush" — whatever simple command you wish to use. Eventually, you will be able to use only the command to stop the dog from barking. Make sure you're consistent; use the same command every time and issue the command every time you shake the can.