You cannot cancel contract at mnt,worst customer service ever

Mtn online/campus Square store worst customer service ever. I need help or advice please .I had a Mont to month contract with mtn,I canceled and returned the router to store 4 months ago,they keep billing me.They are not helpful,onlone they send me to store ,in store they say they can't help me,there is no way to cancel this thing,I reversed my debit order and they put a ban on my name ,they register me on credit bureau. I went to store on Saturday 2 Nov,the regional manager assured me it was cancelled, I had to pay R500,today I'm being billed again.there is literally no way to cancel this thing,I have to pay for services that I'm not using and if you don't pay they stain you're credit and black list you.please advice what steps do I have to take.where can I report mtn ?what can I do?.I cannot pay R500 a month for services I'm not using,not in this economy, never.please advice anyone.