How to tell people not to pet my dog as they are trying to pet her suddenly?

People irritate me when they sneak up on us and stick thier hand on my very large dogs head. She is not aggressive, but is more of a guard dog-personality and doesnt like everyone petting her...she feels her duty is to protect us on outings...........she lets kids pet her but constantly monitors the men around us. How do i nicely inform people not to touch her w/out making her sound like a viscious dog, or w/out making ME sound like an A S S..............