Call the police non-emergency number and report someone in a handicapped parking spot that doesn't have a hanger or tag. Look for Tow Truck numbers posted in the parking lot.
Hit them where it hurts. Their pocketbook, and having to figure out where their car is, how to get there, how to pay, etc... Don't use illegal means when legal means are readily available, willing, and waiting.
Take a picture, and complain to the internet? IT's simpler to take care of legally. Then post a pic of a cop writing them a ticket, or a tow truck hauling them away on the internet.