Mtn is a ****

I canceled my contract with mtn during December, end of Jan they debited my account I reversed the debit order because the contract was canceled, they kept on nagging me and even suspended my services, couldn't make any calls or send sms, went to thier branch and was told to pay the amount that I reversed since it was the last debit order because I canceled the contract in December. I paid the amount and asked them to change me to pre-paid for which they didn't. This month again they debited my account. I don't understand why they do something like that when I canceled the contract and do not even receive data anymore. Mtn is a ****!