black listed

Good day im having a huge problem with cell C, i had a contract with the which i fully paid last year on March 2019, they kept on debitting money from my acc even after i had cancelled and paid my acc, and i kept on reversing their debits, they called several times to ask why i had been reversing the debit i explained to them that my contract was fully paid and cancelled, i just found out that they had black listed me. now i cant do any thing cause of that this is so disturbing and unfair cause i paid my contract in full.