It depends on the TX you are using. I am talking about the Futaba 9C. Once the position switch is set to NULL. The mix cannot be turned off. I also have a JR. It is like Ernie said. Bumping the mix switch will shut down one elev. At least on the cheap JR I have. The 9303 is a different ballgame.
Ernie, on the 9C it will work perfectly as I described. You owe it to yourself to save up and get a 9C. Once you get the programming down you will wonder how you got by without it. Being able to set up a plane in 30 minutes or less from scratch is worth the price alone. I also like the switch assignment. I can make any switch do any function. For example I use a spoileron mix for Waterfalls and Harriers and Elevators. I put this mix on my trainer switch. So when I want the mix I pull the switch. When I am done I just let go. It will spoil you.

David