the correct answer is:
I'm Jedi - may the force be with you. Followed by that weird grunty noise that Yoda makes, or optionally that weird sound Chewbacca makes.
That'll either send them running or bring out the geekiness in them. Plus if they try to do the Vulcan sign you now have your full right to punch them in the face for insulting your beliefs
and getting their "stars" mixed up.