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All I can say is, it's about time. When it gets to the point that we have a word -- troll -- for the people who post nasty, off-topic, ad hominem comments, you know it is time to do something. What you are doing is a small step, but maybe it will be enough, at least for the moment.
When the trolls figure out a way around the new system (I'd give them two weeks to three months, depending on perseverence), how about adding a third option for reading comments: along with "All letters" and "Only Editors' Choices," add "Only letters from members"?