non paying customers .

This won't help but going forward your can tighten up your contract and payment terms. I have been in the vending business to large and small ag based business for over 40 years with up to 1 million dollar contracts and more. Thankfully over that period of time I have learned who the "A" accounts are and who the bad hombres are so I screen my accounts very carefully. Remember, you don't want them all! Some of the things we use are contract terms upon signature, adequate down payments, proper filing of liens, and progress payments or work stops.
Unfortunately that does not help your current situation. I would hire the meanest bulldog lawyer I could find and sue the bastard for work completed, interest and legal fees.
These sort of stories piss me off as it has happened to me and I have had to learn to protect myself.
Best of luck to you------------DR.