i cancelled my contract and they continue to bill me. thankfully although they bill me, they don't direct debit

i cancelled my long running mtn contract in early 2017 - February. I wrote, I phoned I also went into an mtn office. oh and also emailed. all done I thought. Around 6 months ago they started billing me again. so I wrote, I phoned, I emailed. I even had emails back from them. BUT the billing continued. supposedly I now owe them around 5 months of accounts. Luckily they have not done a direct debit on my bank account. BUT they could still blacken my credit record and if they do then it's off to the courts for defamation and they will pay. so why can't they just get their house in order and behave in an equitable and honest manner. Chris ID = ********** 112080