While creating room, you can create room with maxUser parameter 2 and a property say isGameStarted with its value as false. For joining room, you can use joinRoomWithProperties API with property isGameStarted and value false. This way the user will only be joined in a room if the value of this property is false in that room. Now, when game starts in the room, you can update the value of isGameStarted property as true. So even if one user gets disconnected from the game this property value will be still true and hence a new user will never be able to join this room.
Let me know if you have any other queries.