FNB not updating credit bureaus

I paid off my credit card in October last year fully. FNB then snuck in a charge a month later which was not in the settlement amount. I disputed this with FNB through Hellopeter and it was squashed. However, FNB has not updated my credit report and for the last 6 months it has said on my report that I still owe FNB for that credit card. It has knocked my credit score badly. I have tried escalating it with FNB but fear that they will not do anything in a timeous fashion - past experience has shown this to be true. FNB has been great in all other areas but when it comes to their credit card system, they have been very, very poor.