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.
My name is Ioannis and I wanted to share our project with the community.
We have created an online generator of hardware comparators. The comparators are based on an old patent which has expired and is free to the public. The patent describes comparators which are easily replicable with simple gates.
Designing hardcoded hardware components, using VHDL or Verilog, is quite easy and there are plenty of tools to help you with that. However, creating a generator from scratch is a challenge. Finding the design, looking for repeatable patterns and reusable components, placing and routing...
Recent CS graduate with work experience in system administration, software development and scientific research.
Ideally, targeting for permanent junior positions in software engineering with exciting problems and awesome people to work with. Also, open for devops opportunities.
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.