As @Luke mentioned I would immediately put a mechanic's lien on them STAT. Beyond that as has been mentioned, I'd hire an attorney and get that rolling as well. Time is of the essence and you need to leverage everything at your disposal.
I would also possibly contact the Attorney General in the state they are located and look into filing a complaint.
Your example is one of the reasons we require a deposit on large jobs. This is a non-negotiable item for us and if they don't agree, it's not worth our time due to the exposure.