Trabbi,
The issue here is that the agent doing the boarding can't spend too much time arguing with a pax with a wrong boarding order. In my experience, they usually allow them to board just to keep the procedure going. Maybe it works better is two agents are working the flight thus the line keep moving. Fortunately in my local airport people on the bus listen to the announcement and board through the correct door.
Maybe pax need to take a course before allowed at the airport.
Rwy in Sight