Unreturned TV Box

I'm going to go into as much detail as I can so we can figure this out and get it resolved permanently.

We moved about 6 years ago, adding another location to our existing xfinity account. We returned the equipment for the old house in person (2 TV boxes) to a comcast location and all was well.

At the new location we had the option of taking a contract for TV + Internet service that was cheaper than just getting Internet alone, so we signed that contract. The installers came and ran the coax for the internet, and when the time came for them to install the cable box, we said we didn't want it so the techs took the TV box with them. At the new location we have an antenna and some streaming services so we didn't feel the need to have a piece of equipment that we would need to return looming over us (I bought my own modem and router so we could return the comcast gateway as well).

Everything is fine for a year and that contract expires. We choose an Internet only plan this time because there are no more special deals offered. I don't know when it first appeared on the bill, but an unreturned TV box fee started showing up around this time. We call support and explain that we have no TV box at this location. They say no problem, the fee is refunded, and the TV box is no longer on next month's bill.

A year later we discover that the fee has been added again. We call up support and they give another refund. This time we find out that the tech giving us the refund can't even see the serial number on the box that we are being charged for. He also can't determine if it was rented for the old or the new location.

I think that we got the box removed a third time (I wasn't there for this call), and found out that it would be somewhere around $200 to buy it out and stop the fees from popping up again. By the time I saw the fees back on the bill I had a job and figured a few dollars a month wasn't worth the hassle of calling. Now the recently increased unreturned box fee is $8.50 a month and we have deemed that to be way too much for a box that we have never owned.

How do I get this TV box with no serial number off of my account permanently without paying hundreds of dollars?