Rupees / Score are currently stored as a value on the server and displayed on your player's scoreboard on the right-hand side of the screen. They are not tangible items that can be stored in containers, thus can not be stolen.
The exception to this is death. By dying, your Score is reset to 0. If killed by another player, you lose 10% of your Rupees and they are transferred to players within a certain radius of where you died. This number is reduced to 5% for VIPs.
Score is always publically visible, here: http://www.mcrpg.com/leaderboards#
If Score was not public, it would completely change a very important dynamic of the server.