Unauthorized debit orders

I opened a contract with cell c on 2016 then last year between June and July I've made an arrangement for them to settle my account and they debited . Then September they made a debit order of R170 called their customer care the guy who was assisting me said he sees that my account is paid up and it has been converted to prepaid . Suddenly on the 26th January 2019 they debited R500 and R 278.22 then on the 28 another R105 but when I go to the branch they can't see anything because according to their system my account is closed