Cancelation and Refund

I recently found out that I've been getting charged every month for the past year and a half or so for Xfinity. I haven't used Xfinity service in a year and a half or so because I moved out of my apartment. I thought I had canceled it and just now I realized I never canceled it, but I haven't used the service since then. I spoke with several representatives and they all told me that the most they can backtrack refund is about 4 or 5 months, but they told me if I want to get the full refund for the year and a half that I can possibly do so if I file a dispute at my local store. The problem is I don't have a local store near me, they are very far from me, 10hrs away. Does it really have to be in person? Isn't there another way to resolve this? There isn't even a direct number to the store for me to call.