If the place had security, all you had to do was get ahold of them. They are in charge of having such people taken care of. Depending on the property owner/manager's policy, the vehicle would have been towed and impounded right off the bat, or the police would have been called and the same thing done, only with a fine attatched as well. I believe first time offenders get 500 buck fine for illegally parking in a handicapped stall (depends on the state), and if he was using one of the hanging tags that carries a much larger fine in most states for illegally displaying a handicapped emblem.
And for the diff. beteween the license plates and the hanging tags, anyone who can show they will be disabled for a minimum of 1 year (in some places as little as 90 days) can qualify for a temporary hang tag (temp usually lasts about 3 years IIRC). The plates require permanent disability. Either one requires you to take a letter from your doctor to the DMV along with a completed application.
State gov. are starting to crack down on people who misuse those temp hang tags. Because they are so much easier to get than the plates, and because they usually are good for much more time than the temporary disability that gave rise to the need for them, they tend to get passed around or even sold (and stolen). Take the pics to the cops, if they can read the number on the bottom of the tag they can run it and find out who it is registered to. The tags are almost always registered to a PERSON, btw, and not to a specific vehicle. This way whoever is driving the handicapped person around can park without fear in the handicapped stalls. I'm betting the kid with the van wasnt the one it was registered to. (Although he may have been mentally handicapped, I dont think that counts for parking.)