MTN STALLING CANCELLATION

I took cellphone from MTN we had a 24months contract after the period ended they continued sending their data and airtime thus leading me to having to pay them I told them the contract ended so why are they continuing they said it is not an auto cancelation. I said I will stop the debit orders and I did. I decided to go to Mtn nd pay the balance and close the account I told them I wanna pay and close the account to my surprise it was not closed. They continued deducting their money for months I went back to melrose Mtn today I paid their money and requested to cancel this contract as I assumed it was an auto they said I must dial 135 and dial cancelations. I could not cancel the contract as after following all the prompts the phone would go silent. I do not wnt to owe anyone but I am not willing to have MTN stalling my cancelation deliberately. I have nothing to lose if I get black listed I just dnt want to owe anyone unnecessarily. Please assist I need to cancel this account.