forum liability insurance

[Poppy]

Can anyone help. Do you know a company that offers insurance to public forums, to cover against being sued for libel if a defamatory post is made and not removed quickly enough.
Posts may only be up a minute or two, but if seen by the offended party could end in a liable suit.
Has to be reasonable price, as only a small local debating forum.
Closed until we can solve the problem, we want it back so any advice welcomed