It's been happening for years and will unfortunately probably continue to happen. If there is no security at fields I don't see how you can come up with a way to stop it. Some of the bigger fields in Melb have invested in cameras and security such as Essendon, Tooradin and Moorabbin which should help.
Perhaps new aircraft manufacturers could invest in more suitable and secure locking systems, but admittedly this will take a long time to filter through to the flight-line. I note the new Lancair Columbia finally comes fitted with keyless entry.