Overcharged

I canceled my service today. When I went to turn in my equipment, they asked for a cable box that was turned in probably a year and a half two years ago. When I explained that they said they would take that box off my account. I asked if I had been charged for that box and they said yes. I said I wanted a refund for the past year and a half two years. I was told I could only get a refund for the past four months. However, this is not the first time I contacted Xfinity about this box. I had noticed at least a year and a half ago that I was still being charged for it, and when I called, they told me they would take the box off the account at that time. Seeing how they did not do that and continue to charge me I feel that I am entitled to a full refund for all the charges I have received from that box Over the past year and a half or two years