Set a lock on <object> to restrict its use. See 'help locking' for general information about locking and 'help keys' for the syntax and semantics of key expressions.
N.B. In the case of rooms, you are actually better off setting room.free_entry to 0 thus preventing teleportation and then @locking the various entrances. The problem with @locking the room itself is that this can make it impossible to drop objects in the room.