BREACH OF CONTRACT

If my contract ended with Telkom, are they still allowed to hold me responsible for paying month to month? I know my contract ended so I even disconnected the wifi rooter and had Bronberg installed uncapped at my house so I didn't need the wifi with Telkom. I desputed my debit orders as they were still debiting my account monthly. Now they are saying I need to pay R1400.00 because I did not send a cancellation letter AFTER my contract has ended? What is the use then for the contract if you bound to keep paying? How can I resolve this issue?