You have several options.
Hit the Source button on the left bar controls, and switch it to Aux.
If you don't have an ipod connected, the screen will be blue with no graphics, and there will be no sound from the speakers.
Or you can simply push the Volume Button down on the left bar to completely kill all the audio from all sources.
The FM, AM, NAV, USB (if you have a flash drive plugged in), XM (if you have an antenna plugged in) all are constantly active in the background for your instant use if you choose to select them.