Can you expain why you want to do what to do this. That might help someone propose a solution that works for you.
Would it work to tell your WiFi network to "Ask to Join Networks" so that you can refuse?
Some things that come to mind, no sure if they will work for you, but you could create new Network Locations, with different WiFi preferred networks. You could remove the local networks from the list.
If you have multiple Network Locations created, you could use ControlPlane to automatically switch between the Locations depending on whether you are connect to ethernet or not, or based on your location (mostly that would be determined by what WiFi SSID names are in the area).
Again, if you explain why you want the setup you desire, it might give someone else an idea on how to help you.