Over charged

I had a two Standard Bank credit cards I've been paying for more than ten years now. On the 2 May 2013 I received a letter from Standard bank stating that my balance is R381.06. Few months later their lawyers called me remanding money I explained to them that I have settled my account. They told me that I am owing R17000 Ive been paying ever since there is no rest. When ever I'm trying to explain that I have proof they don't care and they are very rude. I tried many times to report the matter to Standard bank I'm told that they don't have that account it might be handled by their lawyers. I don't have the name of those lawyers. I am no longer working I'm a pansioner