Marita,
Actually there is one way to do this without having to manually edit each time, but it would require you prepare the background and hover graphics for a customized menu. You could make both the background and hover menu images feature the 2 pixel wide "separator" on the far right side of the background. The way around keeping the last one from showing (since there are no more menu items after it) is real simple too. You just get the exact width of your entire menu bar - say it is 806 pixels. Since you know you don't want the last separator bar which is 2 pixels, you enclose the menu within a div that is 804 pixels wide (2 pixels less than the menu) and then set the overflow property to "hidden".
Regards,
Brian