Excessive barking is usually the result of anxiety. Depending on the age and the trigger, there are many simple solutions to stopping the barking. If your dog is an inside dog, take a spray bottle filled with water, and squirt him when he barks, using the command, "quiet". Sometimes barking is just a symtom of underlying issues, and there are other humane and useful ways to quiet a "barker". For more tips, and information specific to your breed and situation, go to liveperson and ask for Maya Moon, canine trainer and breeder with 25+ years experience!
You can't stop a dog for barking
Well, unfortunately there is no way to stop a Jack Russell from barking. They are a Fox Terrier and Terrier = Terrorize. They are extremely energetic dogs and need lots and lots of exercise. Tiring them out a little might help.
You can never really stop a dog from barking but try distracting your dog that should stop them for a while, try keeping them occupied.