In Australia, it's common practice to have a throttle lock. Basically a U shaped piece of steel that drops over the throttle rod and is fitted with either a padlock or window lock to secure it in place.
If you have a look at Tony Earle's cockpit photo in the First Flights album, you will see one. It won't stop your GPS from being stolen, but at least the plane will be where you left it.
rgmwa
RV12346