Spammers editing other users posts
 

I run a fansite and even though you have to be a member to post/view there is a section which is open to unregistered users to leave messages. Those posts are on approval. I have noticied today that the posts I have approved are now being hacked and the spammer is editing the original message and posting spam in it's place. I know that the original messages posted where that of fans and not a spammer due to the content so it's not as if the original poster was a spammer, if you know what I mean.

Any advice would be appreciated :)