As a contractor, I have customers (though few and far between) who don't want to pay me once the job is done.?

Sorry to say, but depending on what state you live in the statute of limitations might have expired on your to pursue any legal action against this client. Many states have a 2-3 year limitation on debts like this. If you live in one of those states, you have no more actions to take other than to simply ask for the money.