Disabling USB Supplying for an Arduino UNO

Got a question in supplying and Arduino UNO.

I found out that I can supply the board using an external powersupply with +5V on the PIN +5V. That works fine.

I would like to know if it is possible to disable the "usb suplying" of the board so it just use my external power supply in all time. The main purpose is I would like the board to be off if my power supply is off event if the usb cable is connected with a computer

Thank you very much

well asfar as i remember the arduino will choose Battery over usb power ! ;)

off if my power supply is off event if the usb cable is connected with a computer

oops i didnt notice that

The problem if the power Supply is Off I want to Board to be Off event if the USB is connected.....

But why so it wont be able to do anything with the pc then !

Yes it will when the power supply is on

There is a solution to my problem cannot explain you why would it be a solution but would it be a way or a function that returns me if the power is taken from USB or external?

This might do what you want:

http://www.siliconchip.com.au/cms/gallery/article.html?slideshow=0&a=110648&i=5

It is a way to supply power from your external source... with extra current... and it will shut off when USB goes away.