Hey 007.
Just go in on the Scenario tab, select Force Properties. Then use custom forces and Fixed Player settings. That will make all settings (for example a computer player) locked when hosting the game. You can also go to 'Players' and select the race to be Selectable, that will then give you the opportunity to choose your race('cause Fixed Player Settings will lock the race if a specific race is choosen).
Hope that is what you meant, just write back if it isn't.
Good luck with your map!