non paying customers .

Might also look into putting a lien on their house or property until it's paid off.
They can still not pay you even if a judge orders it, but a lien can cause them all kinds of headaches down the road and in the worst case they would have to pay you off to do anything with the property in the future.

BTW, non paying people are not customers.
We have some "not customers" in our business as well.