So why do you want to turn it off?
If it’s to get rid of the light then just cover over it.
If it’s to minimise power consumption then I’m afraid you are using the wrong board for the job as disconnecting it would work but there are many other things wasting power on the board.
It was just to stop the light illuminating when the Arduino was being used in a dark environment but the best solution is the simply cover it up. Although perhaps it would be nice if it could be controlled programmatically so, for example, I could turn on the LED during the day and turn it off at night.
After thinking about it, if your Arduino board was going to be used in a final solution, mounting it in an enclosure would be a good idea: it would protect your board and hide the power LED where you could use an externally mounted LED as a power or status indicator.