Absolutely disgusted with MTN

I have been trying since February to get my contract at MTN cancelled. I have sent mails on regular intervals instructing MTN to cancel my contract when it matures on the 23 rd of March 2015. AS I have requested to keep my number, I was instructed to phone on the 23rd to change form contract to prepaid. It was never mentioned that the contract will remain and being credited with the airtime again. I have been requesting them to cancel the credits for longer than a month in writing - there should not be a need for me to phone in as well. I have tried to call since the 23rd and only got through on the 2nd of April - after I have been credited with another months fees already. I was told that it will be reversed. Now I am told that I am liable for the month - although the contract was cancelled on the 7th - with more than R 500 airtime being cancelled - who would be that stupid? I refuse to pay for something I have cancelled in writing - all debits WILL be returned.