Usually hamsters bite their bars out of boredom, or if they want to tell you something (e.g. Wheel is too small), but if you feel like you have a sufficient amount of toys, and nothing can be wrong with your hamster, you could try getting a bar chewing spray (available at most pet stores) which tastes nasty to hamsters, and will prevent bar chewing. Another option is to purchase a cage without bars. Pets at Home (UK pet store) has a selection of plastic cages, with barring on the top of the cage for easy access. Another tip is to ensure that your hamster's cage is equipped with plenty of wooden toys for them to chew on! Good luck!
