Sent to debt collector for charges that Comcast has confirmed multiple times that I don't owe

I closed my account at the end of last year, and at the time of cancellation confirmed with the rep that I was up to date on my bill and would not be billed again. My services ended on the 14th of the month, and I received a bill for the second half of the month, after my account had been closed and all Comcast equipment had been returned. I reached out to Comcast two more times, and both times Comcast representatives confirmed that I was being billed for services that I did not receive, and that they would take care of removing the charges from my account. They advised that I did not need to do anything further and that they would take care of everything on their end. A couple of months later I was contacted by a debt collection agency for the charges that were supposedly removed from my account. I advised the agency that I had reached out to Comcast multiple times, and Comcast confirmed that I did not owe this money. After this I didn't hear from that debt collector again. Now, I'm being contacted by a new debt collector, and on top of that they have reported this as an outstanding balance to the credit bureau agencies. Not only is this super frustrating, but this "ghost charge" is now a negative mark on my credit history. I don't understand why it's so difficult for Comcast to handle this simple issue. What can I do to resolve this without spending hours upon hours on the phone or text with customer service reps who have already failed me multiple times?