codegeek has a more useful answer than mine. If you're willing to pay for it, find a lawyer and they'll be able to walk you through it. As codegeek says a threatening letter may be enough to solve this.
If you want to file without a lawyer, I'm not sure. If it was New Zealand we have a small claims court for this sort of thing but in America I think you may have to know a lot about the legal system to sue there.