NAL just general advice. Do not pay any “new” payments as you aren’t using the service anymore and it wouldn’t make sense for them to charge you when there’s no proof of you using the service + you moved out. So the next time you can make a payment, pay the outstanding balance and write a detailed email or letter explaining that you have paid the outstanding balance, moved out and stopped using the service on X date and have cancelled your account. Then remove them from your direct debit.
They won’t be able to bill you moving forward anyway because you no longer live at said address. Technically you should’ve been firm on the phone the first time and said no you will pay the outstanding balance and not pay for any new balances because you aren’t using the service anymore but hopefully this works out!