Contract not cancelled

My contract expired September 2015 and i phoned MTN to let them know that i will not be upgrading thus i will not be renewing the contract.<br> <br> November 2015 I received an sms from MTN reminding me that they will be debiting my account end of the month. I then went to the bank to stop the debit order. I just received units from them meaning that they are still going to bill me after telling them to end the contract. I know will be black listing me cause I stopped the debit order and i will not pay them cause according to my knowledge the contract ended and we don't owe each other a cent. I have been loyal to them for two years i never skipped any payment. All I am asking from them to end my contract and stop sending me airtime cause they will be billing me soon.<br>