MTN N1 City Cape Town Bad service

MTN N1 City Cape Town . I returned a month to month home internet modem on 20 March as i was relocating back to Johannesburg and completed the cancellation forms and handed in the device in the box as i was told. Later on i noticed a debit order on my account and i reversed it . I just called the cancellation department and they are telling me i was supposed to be given some reference to use. How was i supposed to know when they didn't give it to me i do not know their processes , now they tell me they will keep attempting to debit my account for a service they can see i am not even using . This is pathetic i have been with MTN on so many long term contracts since 2010 or so i expect to be treated in a better way

Cleopas