Fraud

I have been told on several occassions that I owe Midland almost a 1,000 for an account I never had at a bank in California. I never lived in California at anytime,but they are saying this account was opened in California and I have to pay it. I never new of this account intill last year and have been fighting with them since. How do I go about getting this taken off my credit? I have proof of where I lived at the time and that I have never lived in California.