On the Ubuntu (version 10.04) I changed the properties of the bottom gnome panel: unticked the expand property, then locked the panel.
The result is a short version of the panel which is very difficult to rest to default using right click (at least I could not do it).
One easy way to reset the gnome panel is to remove .gconf/apps/panel folder. This can be achieved by typing the following in the terminal:
rm -r ~/.gconf/apps/panel
Then you should logout and when you login again you will find the panel is changed to default setting.
Note: By resetting panel all the customization of the panel will be lost.