Dean-
He'll glare the guy down and wrap an arm around your waist prospectively.
Sam-
He'll just stand up and pick you up bridal style, go find a new seat and sit down with you in his lap.
Cas-
Cas will grab your hand and lead you away from the guy. He doesn't fully understand jealousy, but he knows when a guy is flirting with you.
Gabriel-
Gabe'll shove candy into the guy's mouth and grab your hand,running away as the guy chases the bothe of you.