Debit order on my account but the contract expired

I had a contract with MTN paying R360 PM. My contract expired on the 3rd of August 2015, and the last installment was suppose to be in end of August. My contract was cancelled on the 3rd August and my number converted to prepaid but to my suprise at the end of September MTN debited a ridiculous amount of R5222, I went to my bank reversed it, and tried contacting MTN on 808 or *** which will cut my call because the number is no longer a contract number. Again end of October MTN debited R5222, now I have to go to the bank to reverse it, going to the bank cost me a lot of money because I get paid per hour. Trying to call *** or 808 doesn't help because my call is cut short. MTN will you stop this debit order