Unity-2D Launcher is controlled by file called Launcher.qml
.
Whilst this method will work - changing a system file such as Launcher.qml could be overwritten by a future update.
First log out and log in using anything but unity2D(unity2D might crash if you edit that file while in use; in 12.04 Gnome will do).
Then backup Launcher.qml just in-case you need to restore the default settings.
(for 11.04 & 11.10)
cd /usr/share/unity-2d/launcher
sudo cp Launcher.qml Launcher.qml-backup
(for 12.04)
cd /usr/share/unity-2d/shell/launcher
sudo cp Launcher.qml Launcher.qml-backup
Now edit the file and find the line items.appendModel(devices);
gksudo gedit Launcher.qml
Now comment out the line using /* .... */
i.e.
Save - logout and login