I don't know how some 'people' can pull crap like that, and think they are so clever.
Don't get too disheartened. You may have to raise your price (which is great) or require deposits to make these incidents less a problem.
It doesn't take away the sting that some people are just scum, but unfortunately, that's the world we live in.
I like to believe the good outnumber the bad, but I know you have to wonder.
Know you are providing an appreciated service to your customers (of which I am one).
I work in graphic art and decals. I couldn't tell you how many times I've done design work for people to have then take my proofs to another shop and never pay me. I've even done many jobs to completion and have the customer never come to pick up the job. They have the opinion of 'oh, I changed my mind'... could have let me know.
Fortunately, they are the minority.