MTN - Rewardsco problems

I had an upgrade cancelled by Rewardsco ref ***3 in December 2018. But MTN continue to invoice me for the contract even though I don't have the phone and Rewardsco insist the contact is cancelled, I must just visit MTN. Number is ***. When I visit MTN in La Lucia Mall they laugh at me and say Rewardsco are *****. Please assist, or should I just order my bank to reject the debit.