Disable the menu option: ‘File ▸ Eject [volume]’ (cmd⌘ + E) in the Mac OS X Finder menubar.
defaults write com.apple.finder ProhibitEject -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 ProhibitEject -bool false