I recently dealt with a site with the same problem. It seems that they had the worst case scenario. A software company took some posts out of context and used them to promote its product. Many of the posts had been of problems with a particular product (by a competitor), however all had turned out to be user error and were resolved (however the company failed to mention this). Basically they took posts out of context and twisted them (placement) to make their product seem superior. While by itself it falls under fair use, there are things you can do to prevent it legally:
Make sure that your copyright message says, "All content, posts, and information found on this site is copyright of yourdomain.com or the original authors. Reproduction of any of the information found herein is strictly prohibited." or something to that effect.
second you can have members place a small blurb in their signatures to the same effect to ensure that their posts are not used. This is for members who care. Some don't.
This still doesn't totally protect because any attorney would argue that information found on a public forum is fair game. Posting assumes public consumption/availability.
However using such messages is a deterrent.
