Download Asus software for the gpu and see if there is an option to turn it off there. otherwise, I can think of 2 options:
1) get some tiny pieces of black tape or something and place it over the LEDS
2) remove the cooler and disconect the LEDs manually. WARNING this will void your warranty.
--Update--
I don't think there is an option to turn off the LEDs in the software (still try though) so I think your best bet will be to cover it with something you won't notice, so black tape or something like that.