I use the tube from an ole' bic Pen and cut off about 1" in length. Drill a hole in the firewall the size of the pen and glue the tube into the firewall with about 1/8" sticking out the front.

Run your looped wire through this to a servo with an EZ connector. Thread the fuel line through the loop, and set your throws on the servo (a computer radio makes this very easy) so that at full throttle the loop just sticks out the tube, and when you pull it back, it pulls the line into the tube and pinches it off.

Only thing to be carefull of, is before you shut the plane off, make sure you open the throttle releasing the fuel line from the tube. If you forget, you will have a hard time refueling.