The Dubro smoke valve may work. All it does is pinch a line anyways. You could put that between the carb and the tank and use it to cut off flow. Just would require another servo. Just an idea
Or if you have a carb just use that. Thats what I use on my little Shrike. Just throttle down and use trim to cut off.
http://www2.towerhobbies.com/cgi-bin...?&I=LXD625&P=7