I just got in the ACOG I'm gonna use in my M16A4 build. I am still waiting on the rifle but looking at the ACOG I don't see a way to remove the Killflash without taking it off the rifle.
A killflash is a screen that goes over the glass end of the optic, so it cannot 'flash' a reflection of light that anyone on the bad end of the boomstick might see.