cancelled account

I cancelled my account in August as my contractct was due in september I did not want to upgrade my account so I called custer services they informed me to write a letter to retentions of which I did its now december and they are still billing me I put a stop order on the account. They now tell me I'm in arears and they cannot cancel my account unless I pay the arears I haven't used thas I got a new contract with vodacom as cellc signal was giving me problems. I want this resovled and I will not pay the arears as I was not at fault I did my part by sending the cancelation letter