you have to make a risk assessment at the time. This is another one of your questions with no blanket/simple answer. If something is blocking a door that’s lightweight and easily moved to the other side of the vestibule when needed, on a quiet service then you would use common sense and leave the passenger be. Maybe have a polite word with them that it could become a major problem further up the line if the service becomes busy. But if it’s a large, heavy object that’s leaning agains a door, on a packed service, then you’d probably have to refuse on the grounds of safety. I won’t let bikes on a 158 behind the driver at busy stations for example. It can be too dangerous if I need to open the door mid journey. Each case is different.