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Remember, anyone can sue you for anything even if they're wrong. A disclaimer will let readers know that your not responsible for content provided by others. Yahoo and other have 'not so favorable' content all over the place but, they don't take it down just because someone asks.
What you need to do is open a dialog with the company threatening suit and ask them if this post exposes copyrighted material, material covered by the DMCA, or if it infringes on any IP they may own.
If it is, I would suggest they give specific example of how it infringes then consider removing the post. If they just don't like that someone said something negative, then they are SOL. It's call the First Amendment to the Constitution, Freedom of speech.
Let them know that if they want to go after the poster directly, then have them get a court order to expose the posters information stored in your system. That's all your obligated to do unless there's a copyright or other infringement.
Hope this helps.