TFG Payment Harrassment, but payment was made already

I have a TFG account which is paid every month by debit order. Last month they did not deduct the owed amount and started calling me to pay it. I got nowhere with their debit order department and eventually paid it manually. But even after payment I keep getting calls requesting that I pay the account, up to 5 calls a day. Explaining to them that payment has already been made doesn't help, I just get another call later in the day requesting the same thing. How one of the biggest retail groups in South Africa can't even sort out their internal departments is beyond me. I am at the point where I just want to close my account and be done with it.