Disable the menu option: ‘Empty trash’ and ‘Secure Empty trash’ in the Mac OS X Finder menubar and command + click trash contextual menu.
defaults write com.apple.finder ProhibitEmptyTrash -bool true
To make the command take effect, you need to restart the Finder.
killall Finder
Revert to the default setting:
defaults write com.apple.finder ProhibitEmptyTrash -bool false