WORST; WORST service EVEER!!!

The worst service EVER!! Have NEVER experienced anything like this! I had an account with Cartrack which I cancelled. They informed me that I cancelled too late and that I need to pay one additional premium. This I did - sent them proof of payment numerous times. They then gave my details to a debt recovery company - Nimble. I sent them proof of payment numerous times! Still reflecting I owe money! PLEASE HELP - do not know how to handle such incompetency!!