Option for REAL private chats (i.e. users not in the chat don't see it)

So far I have only figured out a way to prevent a user from posting comments in a private chat. They can still go to the node, see the chat as it's happening, and just sit there if the wanted. That really isn't a private chat at all. It's a limited participant chat, sure, but certainly not private.

How do I set it so that only those users involved will see the chat? Thanks.