if you're playing the IGC, use the config utility and leave 7 of the 8 slot to 'none' and choose your country in the 8th.
If you're playing the "normal" GC, just open the scenario file using notepad and erase 7 of the 8 nations - it will start with a 3-letter tag (i.e. FRA= .... or ENG=....). for the last nation remaining, just put in the 3 letter tag you wish to play.
cheers