How can I remove letter-spacing for the last letter of an element in CSS?

I would call this a browser bug, actually. The spec says it's the spacing between characters, while your browser (and mine) seem to be changing the spacing after characters. You should submit a bug report.

You can set your element to have a right margin of -1.2em, which would counteract the letter spacing.

e.g.

.menu-header-selector {
  display:block;
  letter-spacing:1.2em;
  margin-right:-1.2em;
  text-align:right;
}

To answer your question regarding pseudo-selector, there isn't a per character pseudo-selector as far as I'm aware. (EDIT: Scratch that, there's the :First-Letter selector, which Jonas G. Drange pointed out).

EDIT: You can find a basic sample here: http://jsfiddle.net/teUxQ/