Man that sucks. My advice is, do everything in your hand from a legal perspective - consult a lawyer, it should be free - show them you're serious about your work.

If you still have no answer, expose them! Write a blog or an article about their malpractice. Publish the code you wrote for them if you can. Perhaps, that will draw their attention.

Put an equal amount of effort in proportion to the amount of money you're owned. If your effort becomes too great, drop it and spend your time elsewhere.