I have a message board discussing a brand of boat. Some of my members are unhappy about a warranty issue and are venting on my site. Nothing they have said is untrue but they are certainly not being kind to the company. Can they take legal action against me?
It's almost impossible to do that, you'd beter ban there IP adresses so the can't come back easily! and otherwise start your forum with registration that has to be confirmed by a moderator! (if you dont have that many users)
Moderate all your forum posts to be on the safe site. All the negative feedback may harm your business.
Liability of a forums site owner is a very long conversation. Chances are the company will never take legal action against you and request a takedown notice at best.