Woops. Anyone who can tell me how?
Which properties of the desktop do you want to lock down? What files can be put on the desktop? The desktop picture? The Dock?
The Desktop picture. It's part of a prank on someone with very small ears.
Navigate to hard drive/Users/user folder/Library/Preferences, then choose Get Info on com.apple.desktop.plist and check the "Locked" checkbox. The change takes effect immediately. Just unlock it when you want to be able to change the pic again.