How can a person deal with overzealous ideologues?

Suppose that an overzealous follower of a particular belief system constantly monitors and polices the behavior of other followers. When he sees what he believes to be a failure by someone to live up to their ideals, he attacks that person publicly, trying to shame him into proper behavior. What is the proper response if I am attacked by this overzealous follower in public? What if the attacks are private? Should I respond if my friends and acquaintances are attacked?