We currently use the reputation system for our subscribed members. I might try having a look around for that.
What I'm after (exactly) is like what you can see on this page:
http://www.wowhead.com/?quest=7631#comments:id=23407
You can see the green text, and things like Rating: +30 beside some of the posts. Doesn't show a deleted one, but when you get enough negative hits, it basically soft deletes the reply.