My first hamster was a real bar chewer too!
It's not really something you can completley stop the hamster from doing, however,it may be a sign that the hamster is bored, it's waring it's teeth down, or it may just like to do it!
When this was happening to me, we put scented chew toys in the cage, and although she still chewed the bars like mad, it wasn't as bad as it used to be.