Charge interest on zero balance

Fnb charged me interest on a zero balance revolving credit facility. I settled it on the 29th September and on the first they charged R420.98 interest ,for what because the balance was zero .

I called and they said they will escalate the issue and get back to me in 2 days .they never did instead they changed that revolving credit into unsecured revolving loan and never gave me my money or communicate with me