Change Windows TEMP folder to a different folder; In my case, R:\TEMP\ (a ramdisk, because SSD)
Play ArmA3 for a while, or until tmp or tmp0 is running. Might have to join a multiplayer server running battleye.
Quit ArmA3, check for running process name tmp or tmp0.
Steam will say that ArmA3 is still running.
Kill process tmp/tmp0.
Steam finally thinks you've closed the game.