Don't. You don't have to act on my behalf to protect me and I wouldn't dream of asking you to do so.
Whether a post is libel is not something you or I are to determine. And using "libel" as an excuse to delete a post is only a very tiny step away from "I deleted it because it wasn't true", which quickly becomes "because I don't think it's true".
Either don't do it, or move to a site that'll let you do it. Period.