MTN attempted to ***** money from me

I cancelled my account at the start of October - an account that gets paid in advance. I was told that my cancellation will be processed and my account will be switched over to prepaid by the end of the month. Woke up this morning and see that MTN debit ordered my account. I immediately had the debit order returned and revoked MTN's debit order mandate with my bank. I consider this as attempted theft by MTN. I am not paying a cent more. If MTN can prove that I owe them any money for October, then I will gladly pay for what is owed for October, but I am not paying because MTN's staff couldn't cancel a simple month-to-month cellphone contract. I would appreciate it if MTN would contact me to sort this out before I get legal action involved, which I'm very tempted to do as this is *****.