FNB is *****ing money from the credit card

I am very disappointed from the terrible and useless service of FNB. have a credit card from FNB with 55 days interest free. They charged me interest after 45 days of purchase. I settled 45 days after purchasing. I went to the branch and they advised it will be sorted and they will phone me. Never have heard anything from them again. They are *****ing money from their clients and nobody is actually able to sort the matter out. If you go to the branch they tell you they need to phone the credit department but surprise this lady is never available. The service is really unacceptable and I want to cancel my credit card and all my other facilities with them. I went to the chat to get help, the consultant did not even read my question he just copied and pasted a standard sentence. I want immediately my money back you owe me!!!! Even the lady at the bank advised that they are not supposed to deducted that but nobody seem to care about their clients.