I believe you can disable it in your privacy settings, but it's not really necessary. We don't much care what people do in private as long as they're not breaking site rules such as using PM's to harass and attack other users.
And in those situations the attacked party can simply invite a staff member into the conversation or report the conversation if the conversation was set to disallow invites. Once it's reported (via the actual report function in the conversation), we can see it.