You can turn off the 1/3 Stops for ISO. It is in the Custom Function Menus. I can't recall which one, as I dont have my Camera in fron of me.
maybe I'm not looking hard enough, but the only thing I see that's close is "CF6- Exposure level increments 0:1/3 stop 1:1/2 stop" but that isn't doing anything to the ISO. There is also ISO expansion, but that only disables ISO3200 not the other ones.