It depends on my current mood. I sometimes just glance at this/these people and then ignore the person(s), since I consider that a lot of people that criticize the others "just for fun" aren't worthwhile and are often pathetic and actually insecure (and when they're in group they love to annoy people), or, if that really anger me, I'll stand up for myself. A ready reply is needed, and, even if I can look timid and else, I'm another person when angry x)