I think it's easy on both ends for lines between constructive criticism, disagreement, and flaming to become blurred. Everyone has felt strongly about something from time to time and reacted rather irrationally. Everyone has been on the receiving end too. What I usually do is speak to the person and find out their true intentions. If they were not trying to be rude I just agree to disagree.

If they continue attacking me after I've tried to make peace, then I know they just don't care.