powering off the RAMPS board

My i3 Prusa style printer is running great these days but I want to make a modification. Some time ago I added a power indicator light, a power switch, and fuses to the PSU - which all work fine. The modification I want to make now is this: when I turn the power switch off, I want everything on my printer to lose power, the Arduino, the RAMPS board, everything.

As things currently stand, when I turn the power switch off, the USB connection to my PC provides power for the Arduino which (please check me here) provides power in turn to the RAMPS board. Since my hot-end cooling fan is pulling 5V off of the RAMPS board it continues to run even if I turn off the PSU. So it runs nearly 24x7 for no good reason. I realize I could pull out the USB cord but a) I'm lazy and that would mean lifting my hand and moving my arm; and b) whenever my PC loses the USB connection Cura and Pronterface revert to connecting to COM1 instead of COM3.

Ideas that I've considered:
1. Add a relay that connects the USB to the Arduino board only when the PSU is on.
2. Add a 12V to 5V converter to the PSU and connect the hot-end fan to it.
3. Hire someone to pull out the USB cord for me and then pick COM3 for me when I next run Pronterface or Cura.

In case it matters, I will mention that I can't foresee ever wanting to run the printer stand-alone (i.e., without being connected to the PC).

Any suggestions?

First, make sure that your printer can function without the USB cable connected, especially if it has a graphical LCD. The 5V regulator on the Arduino is overloaded in many Arduino/RAMPS setups, so your printer may be dependent on 5V power from the USB connector to work properly.

I think the simplest way to make sure everything turns off when you remove 12V power is to cut the +5V wire in the USB cable.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

You could replace the 5V fan with a 12V fan and connect it directly to the 12V PSU.
Having a simple 5V regulator will do the same, but 12V fan is easier.

You can buy these 5V power supplies from eBay,

This can be used to power your arduino too, if LCD fries your 5V SMD regulator.
The same regulator 7805 can even be soldered directly to the Arduino, once you figure out them legs. (In, Out, Ground)