Can't return what I don't have

When returning a piece of equipment last year it was discovered at the xfinity store by the staff that I have 4 cable  boxes on my account instead of the two that I actually had and was  keeping. I explained to the customer service person that I don't have those devices. He seemed to be of the mind that what i was saying was inconceivable, and he could do nothing until I brought them in. 

I ended up leaving without satisfaction and assumed at some point they would be "paid for" and dropped from the account since he agreed that they had not been active as far as his records spanned.

I see that i am continuing to pay $14 a month for the phantom devices. What is the procedure for getting these removed from my account?