Toxic behaviour

It's been 5 months since I returned their month to month contract router at Northgate 42A Northumberland Road Randburg because I didn't need their services anymore, now it seems I'm held ransom for reasons only known by the entity every month my account is debited and I'm receiving a bill for something I don't have, I have no ties with MTN what must I do to be left alone this is becoming abusive and destructive I will never ever ever ever do business with this company never ever