Can someone say me the code When someone types /clearchat then the Chat will be cleared? That will help me Much =).
Since the chat history is saved clientside, the only way I can think of to "clear" the chat on a non-modded client would be to spam every player online with blank messages that push past messages out of the chat history. Doing anything more sophisticated than that would, I think, require a client mod like Spout that gives the server more control over what the client "sees".