Why I'm I still being charged after I moved to a new address with equipment returned

Hi,

I moved to the new address around Septermber and the modern was returned at the time. Why my old account is still not canceled and new bills still shows up?

I called customer service for a couple of times and several representitives said they will take care of it. The bill was waived once, but they start to show up again. I want to know exactly to get it canceled and have the bills waived for the recent two months.