Basically, I agree with the others above. You don't have a lot of choice.
Now, moving forward, start this TOMORROW MORNING!
You MUST impose a 'Quote' process where you can provide a quote before you do any work. Now, I know that's easier said than done. But hold on ... this will work. And if you want to succeed, it MUST work in your business.
You start with a Quote and tell people that there is a CHARGE for that quote. That means, you check it out... you run all the numbers and tell me that it's going to be $500 and I say forget it... you STILL get paid for the quote.
Now if I bring it back to you (in XX days... usually 30 days) you will DEDUCT the price of the quote from the job. That's because you would have had to come up with the quote to get the job, right? So that's only fair.
I operated an Auto service business. We did a diagnosis and quote. The customer signed the work order... and was asked to pay for the quote. It was 5:00pm and he said he was coming back in the morning to get the job done. We still insisted that he pay for the quote. When he asked why, it was simple. People DIE! We did the quote... we get paid.
Don't mean to be a pushy guy... but you MUST have a process that you approach all your jobs with. That way, when people (sorry, jerks like this guy) don't want to pay... you've got it covered.
Hope this helps! Remember.. you gotta start this TOMORROW MORNING! Got it?