Refund being refused

I put my account on hold and xfinity randomly charged me over $100 for a month. I have been out of town, nothing was plugged in so no data was used. I have been back and forth 4 times with customer service asking for a refund and have been told it would be approved but it is denied each time. They have effectively stolen from me. Does anyone know which agency I can file a complaint with? I live in Washington State.