I've cancelled my service 3 times and am still being billed monthly

This is so ridiculous at this point. I've talked to 2 agents over the phone and one over chat. All 3 have assured me I will not see anymore bills and yet every month, they arrive in my inbox with a new monthly charge in addition to the past due amounts of all the months since I've cancelled the first time, 3 months ago. In fact, I am owed a credit for the remainder of the month after I cancelled, and again, all 3 agents have assured me I will see that credit in my bank account. That has not happened. I've changed ISPs and am not using any Xfinity services, the agents have said my account is closed. Yet the bills keep coming. What else am I supposed to do? I'm worried it'll be sent to collections and impact my credit score.