Nimble owes me money, shocking/no service

My account was paid in full. Actually overpaid as the lady gave me incorrect outstanding amount. I still get sms messages to say that I have not honored my R1000 monthly payments. After paying the full amount they go and update my credit profile with the full amount still outstanding. It clearly shows the people that work for this company has no idea what they are doing and now impacting on my mission to increase my credit score. Must I now start sending letters of demand to get my R500 back? Do I now list them as bad payers? Someone please get back to me. You keep me holding for 23min and I get disconnected. Shocking shocking service