Billing and Support Nightmare

Hi, 

I am looking for assistance in getting a bill disputed. I canceled my service over 6 months ago and was continually charged, until the service was "disconnected" because of lack of payment. Even though I canceled multiple times months prior. During that span I have called customer service numerous times and have been on the phone with them for many many hours. Every single time I am on the phone I have been told I would be refunded for unused service and have my plan cancelled when in reality I was still getting charged and the refund was never issued. Now they are sending the full balance to collections even though I was promised it would instead be refunded and would stop growing. Each time I call, chat, or go in person to an Xfinity store we have to start over from square one, I am promised a refund will be issued within a few days, then the refund never appears and the bill continues to grow. with this being sent to collections I am worried  about credit score impact and Xfinity losing the ability to deal with it. Does anyone have similar experiences and any ideas on how I can get Xfinity to honor their promises?