Losing payments

I made payment on 24/12/2020 via internet banking from my NEDBANK account, unfortunately I quoted an incorrect reference. Now my payment has been allocated to somebody else's account. I have sent proof of payment together with a statement that I made an error. Nothing has been done, instead I constantly receive threatening smses.to make matters worse I am trying to cancel as the contract has expired, however no such luck they claim I have an outstanding balance. What am I supposed to do. I can no longer afford this contract. Nobody is helpful at all.