The reason this can't be done:
Saving is Clientside. When a player joins, their game loads all your bricks so they can actually play on it. Since their game now has all these bricks loaded, their game can save what it has. A server can not tell a client not to save the data the server sent to it. Even if Badspot put a block like this, someone could find a manual or alternative way, since all it is is saving the data given.
Meh, I lost where my full point is.