How to tactfully remind others that a room is off-limits?

Putting a no-entry sign directly on the room does not express what you want to say more directly.

If it is inconvenient to post such a sign, after the other person enters the room and wants to open the door, remind him or her that it is best to knock on the door and ask if it is inconvenient.

After all, everyone has their own privacy, and everyone understands it when it comes to this. Most people would not enter other people's rooms easily. After all, they all know that it is impolite.