Libel or Slander?

Do we as agents have to take what people say about us?  Where is the limit on how we can respond to threats of adverse blogs and false accusations?  I certainly don't want to get in to a contest of who can say what to who, but it doesn't seem fair to be in the public target range for whatever their thoughts and feelings are.  I'm the first to admit it if I have truly made a mistake, but how do you cope with people who are saying these things about you that you know aren't true?