Wrongfully pursued

Hello everyone, I am looking for advice on a situation that I have found myself in. About 8 months ago, I got into an altercation with my brother-in-law. We were near his car, and he ended up throwing me against his rear passenger window, which broke. He subsequently called the police and claimed that I had gone crazy, purposely breaking his window. The police gathered out accounts of what had happened, and that was that. About 3 months ago, a collection agency began calling me and demanding that I pay the 1200 dollars for the damage to his car (apparently he claimed that I also kicked his doors, and had some dents paid for in the same claim). I was never contacted for my side of the story by his insurance agency, which is also my own agency (State Farm), and when I told the collections agent that I was not responsible, I was treated as though I was dirt and told to fax them evidence that I was not responsible. I called the sheriffs department to retrieve the police report, but they can not find it. Therefore I have no proof of anything, and neither does the collections agency, and yet I am being relentlessly pursued. Is this not illegal? What can I, or what should I, do to get them off of my back?