How can I limit the GUI choices in the drop-down menu at login?

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http://askubuntu.com – Having installed GNOME-shell, Cinnamon, and GLX/Cairo-dock, the drop-down GUI list at login is now quite long (3 GNOME options, 3 GLX options, 2 Unity, and 2 Cinnamon). I only plan on using a few of these options, and it would be great to remove the other clutter from the menu. So far, I have not found a way to do this. Is it possible? (HowTos)