Hi Dedi,
You really should ask your own lawyer about this. A few years ago, we sued someone over false claims that she made about us, on her own forum.
If you use your own forum and post messages to spread lies then you might be liable. Check out
http://blog.nj.com/jerseyblogs/2009/...sts_too_m.html
and then check out
http://www.reuters.com/article/2011/...7565Q520110607
Maybe what you should do is check to see if the statements are true yourself, and then for the overall integrity of your forum, remove the posts or at least ask the poster to backup his statements with links.
Good luck!