Account closed but accumulating balance

I had a jet account which I have paid in full and closed last year in June, promised to send me closure letter in 24hrs. Months later I found out that the account want never closed and now in arrears of R374, I queried that with them, they promised to fix the error but never done, I lodge the complain on hello Peter in April, jet called me to rectified error but today I check my credit score, I still owe Jet and the never sent me the letter in April as they have promised..... I am so furious