There is a spray called Bitter Apple, usually used for dogs and small pets. Apply it to the bars of the cage and ur hamster will never chew on the bars again. Make sure you do not get it on ur fingers because it tastes horrible and it takes forever to get it off.
Make sure that he has something else to chew on. They make things especially for this purpose that you can purchase at pet stores. Hamsters need to chew to keep their teeth worn down. He's just doing what nature is telling him he needs to do.