I'm seeing a lot of this; people parking in handicap spots with no handicap sign in their vehicle. So I printed up a bunch of messages that read:
"You must have a handicap sign displayed in your windshield to park in a handicap spot. If you don't have one, don't park here. The next time you will be reported."
I fold up the message and stick it under their wiper blade. I have been noticing less violators, but I don't know if it's just coincidence, or if my handing out of these messages is actually getting through to these idiots...do you see a lot of this where you live?