Unauthorized debit orders

I had an account at Cell C witch I cancelled in April 2018 after my 24 months were over. I paid the installment due cash and sent proof of payment to Cell C. Sine October 2018 NuDebt put in unauthorized debit orders on my account, witch I could Reversed...Unlikely for me, They put one again now for R818 and I cant reverse it. No help from the Call center whatsoever!!!!!!!!!!!!!!!!!!!!!!!!!!!THIS QUALIFY AS THEFT!!!!!!!!!!!!!!!!!!!