If you want to turn off the Recent Documents feature in Ubuntu, all you have to do is create a .gtkrc file in your home directory.
touch $HOME/.gtkrc-2.0
Then add gtk-recent-files-max-age=0 to this file.
echo ‘gtk-recent-files-max-age=0′ | tee -a $HOME/.gtkrc-2.0
This is documented here.
This will also work in any other linux distribution running Gnome.
Enjoy!