Clarification on what can get you banned: Insults

This seems very vague in what it means. Do you have to consistently do it, do certain terms and words bear more weight than others? How do you handle situations where the person obviously instigated a response from the player? As someone who advocates players are free to speak their minds within reason, it seems rather silly that you can be banned for defending yourself against people trying to get a rise out of you or troll you.