Termination of contract

I had a cell c account and I closed it in December 2021 after paying the outstanding amount, in January the account was still open and they told me to pay a further 390.00 because the time I closed the account the statement was already generated. I paid the amount...and after a struggle I got a confirmation that the account is closed. Today I received another statement stating that I owe cellc 2800.00 so please help me anyone. I don't want anything to do with cell c anymore. How can I get them to get the hell our of my life????