I always find it strange how people are seriously concerned with trolling. I understand that children, the emotionally immature, and the psychologically ill are actually harmed by trolling. However, those people are already at risk of being harmed by anything approaching verbal abuse. These are the people who have more important issues to deal with than trolling. These people are irrelevant in this context.
The people who matter to trolls are those who can't take a joke, who are super serious all the time; they want to be mad about something. Trolls merely give those people what they want. Flame wars ensue. Purposeful trolls are social pranksters and are conscious of their trolling; these can be reasoned with and are thus not serious issues to fora.
The actual trolls worth worrying about are those who troll yet know not their trollish behavior. These people begin arguments over trivialities and get called jerks/dicks/assholes. These people are much harder to reason with, because they will only listen when they are emotionally sober; bringing up their trolling just gets them deep into another argument. You can't stop trolls; they have to grow out of their own emotional immaturity.
Thus, as the unconscious trolls are themselves a subset of those harmed by trolling, they can and do harm others without understanding what harm they cause.