I think this is a question to ask a some SEOs about, because I'm not sure if you should be getting penalized for those pages. Generally google is pretty smart about knowing what a message board is and not penalizing you for those pages. Let me ask around about this.
If you want to limit those pages you need to make a robot.txt file to specify what you don't want indexed.
http://tools.seobook.com/robots-txt/
