A resistor in line with the inputs will drop the input voltage as much as you want. The volume can then be turned up all the way and not be very loud. It won't hurt anything and if switched would enable you to turn it off and back to normal when you are there. Anyone using it will simply think its not a very good (loud) stereo. Total cost, a couple resistors, next to nothing.
There are a couple ways of doing this. Least invasive but also least effective is to put the resistor in line with the RCA connectors of whatever is plugged in. Problems with this are you have to do it for each connection, and it doesn't protect in case they plug their iPod into a jack. The better but more involved solution is to open it up and follow the inputs to the selector switch and install the resistor at that point. This way no matter what input is selected they all get trimmed. That is all you're doing by the way, trimming the input, same as the volume control does. You are basically just adding another (fixed) volume control.
This is the more elegant solution but it does require a bit more study and being able to follow the wires inside, and then you would have to figure out how to switch it. Anyway, there's your options.
