Same... I have had a lot of threats and a couple of attempts, all of which failed quickly, because the laws for online forums protect the owner unless the owner has knowingly breached another law, ie. copyright, trademark infringement, etc. Otherwise, many a court case already exists as precedence that the owner of the site is not responsible for what another person writes on their site, defamatory, libel, or not... the owner is not responsible for that of others.
You as the owner cannot prove or disprove whether what someone says is libel or not. Another party would have to first subpoena your data for that users details, then take that user to court, not you, for proof or otherwise of their comments, then if court ordered that you as the owner remove the content as it is proven libel or otherwise, you do so. If you didn't, then you would be breaking a law.
Until then... precedence of existing cases is quite within the owners rights and puts the onus on the user writing the words, not the site owner.
If it was the other way round, Facebook, Twitter, every forum and blog that allowed comments, wouldn't exist.