TimBernersLee describes the concept of PublicPlacesOnTheInternet?. But anything public that can be posted to is not SpamProof. (As some have observed, WikiWikiWeb is vulnerable to Spam also.) The popular solution to this problem is to create a nominally public place, and then add a ModerationPolicy. Generally someone is chosen to be the moderator, and they choose which submitted messages get to appear on (or stay on) the public place in question. Actually, even PublicPlaces? in the real world can suffer from this problem. (If you want to avoid some of those strange characters on the street, go to the mall.)
MessageReposting is a way to allow moderation, but without tying the ModerationPolicy specifically to the location in which the message is posted to or read from. It enables each user to be a moderator, and any user can subscribe to any other user's moderation policies.