Now this is a simple hint.
If you work with an application that has many windows that you keep moving and resizing, you know that missing your target and clicking on the desktop will activate the Finder. Those who work with Strata 3D on a single 17" monitor know what I mean.
You can use TinkerTool to add the "Quit" command to the Finder's menus. When the Finder is not running, clicking on the desktop won't do anything. You still get the Desktop Picture, but you won't deactivate the front app, unless you activate another running app (better hide those).
To get the Finder running again, simply click on its icon in the Dock.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030418122833718