Well, I think that if the whole tree of replies get marked for the reaper you might as well just kill the whole thread permanently. Otherwise, what you describe is about the best idea I've heard. You preserve it for posterity if they look real hard, you purge it from easy viewing and you leave well written comments in the system. That is pretty much a win anyway you slice it...

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$you = new YOU;
honk() if $you->love(perl)