That's going to be one for the lawyers to resolve for you.
Depending what is being stated on the site, if you have a decent raport with the site, you may be able to have the comments removed. If things are flat out lies, you have a better chance, but you need to prove them.
One of the beauties of the web forums is that there is a lot of room for people to make comments and post experiences, but just like a newspaper there can be legal suits brought against the poster for invalid content.
Where the legalities are however, that's something that a lawyer would have to determine - If the issue is with a particular company, I'd THINK that you would file the suit against the company, rather than the location of the data - but again, legal advise is recommended on this, I am not a lawyer.