Still Being Charged for a Cancelled Contract

I cancelled my contract with Cell C just over a month ago and actually went into their branch to settle the account in person. I paid just over R1000 to settle the account and they told me that it would be cancelled and that no more was required from me. Two days ago I was debited with over R500 and then, when I went back into the branch, the same person with whom I settled the account told me that after the additional R500, I STILL owe R800. This all after I have been told that my account is CANCELLED?!