Moderation is essential because by publicly making an example of trolls, you can change the culture to prevent it. It's the same theory as patching up broken windows to ensure that worse things don't happen.
By shining a light on these groups you attract others to it. This is why (I personally feel) the extreme right and extreme left exist today in politics. When you highlight (even to shame) a group it galvanizes them and brings other loonies to their cause.