If you had a box and did not turn it in, regardless as to whether or not it is active, plugged in, and/or being used they can charge you. If you have the reciept, and show proof you turned in the converters you can charge them back for the months you had to pay for the converter rental after you turned them in or use it as a credit towards your bill. If you never had a box and these are just a/o's they can not charge you per the communications act of 1996/7.
This is CABL.com posting #134039. Tiny Link: cabl.co/mI15