If you overpaid them, they have to return the surplus--your accidental overpayment does not give them any right to money. While they could apply it as as a credit vs. future bills, that would only be with your agreement--you have the right to get the money back. If they will not return it, your recourse is to sue the cable company--you would need to prove in court what you owed and what you accidentally paid (and therefore how much they owe you).