I actually used that feature on my recent trip quite a bit. Just reset the fuel gauge each time that you fill up and set your "miles per tank" to something reasonable like 240. It will pop up and give you a warning that you are getting low, and leaves an icon on your navigation screen that will give you fuel stations along your current route.
But to turn it off just set the "miles per tank" to zero. You get to that setting by tabbing to the screen that looks like a dashboard and shows the fuel gauge on the right side, and press the reset button on the bottom of that screen.