Ubuntu One starts automatically while there is a folder called "Ubuntu One" in your $HOME, thus delete that folder if you want to prevent Ubuntu One from autostart.
Then if you want, you can also restore the factory settings by deleting the following hidden folders from your $HOME
Please note that you don't have to delete
/etc/xdg/autostart/ubuntuone-launch.desktop file, in fact if you do so you might be into trouble when you want to use Ubuntu One again, however you can rebuilt it with:
sudo gedit /etc/xdg/autostart/ubuntuone-launch.desktop
by pasting the following and saving changes:
Exec=/bin/sh -c '[ -d "$HOME/Ubuntu One" ] && ubuntuone-launch'