Account was not cancelled despite giving notice

My account with telkom was cancelled in Dec15 and I have all the proof, despite this, the irresponsible people have not cancelled my account and it went to a R 1200 owing by me. I only picked this up when I went into the store to open another new account. Despite calling and going to the store several time nothing has happend yet and I dont want my credit to be in a black mark coz of telkom's incapable employees. Kindly help