Ugh! There are some total plonkers around I've come across my fair share in my working life too.

If you have designed something then you should have copyrighted it at the very least.

Can you record their telephone calls to you? If not get one of these handheld dictaphones. I've got one which has a facility to record telephone conversations via a little earpiece. Surprising how some people back down very quickly when you tell them the conversation has been recorded and you will put the matter in the hands of a lawyer