The reality is humiliation doesn't work to change behavior. But natural consequences do.
Check the law in the area you live. In my city it's against the law to park on the sidewalk portion of the driveway. Take pics and email or snail mail them anonymously to the police. This was the tactic before cell phones, where disability advocates used Polaroids of nondisabled drivers parking in protected parking spots. to document and send in the info to the police or in this case the DOT would also issue tickets, and tickets can be issued by mail. They were glad to do so. Tickets bring in revenue and It took zero work on their part.
To clarify, these are people without proper handicap accessible parking tags. Not just someone who doesn't "look" disabled. There are plenty of invisible disabilities.
It might be tempting to photo a person in a wheelchair or other visible disability being forced out into the street, but in the past they were often given tickets...so it's a bit sticky. I am not sure if that is still the case.
Another problem with the sticker is that it doesn't teach other people:the audience, anything about why it's bad to park there, which is actually very important. Most people just don't know. Or don't think.
Vigilantes generally only accomplish short term attention, whether negative or positive and not change. Advocates both show and tell, must come across as fair and reasonable. It's all a matter of looking inside and ask yourself are you wanting to vent your frustration or put it to work.