If you are only interested in discussing a topic with selected individuals, why not start a PM thread wherein all responses are sent to and received by the same people? Not sure if that's practical, but the only reason I can think of for having a thing like that set up where it can be viewed by everyone would be to take advantage of the general forum's ease of use without having to "put up with" views, objections, questions, etc. that you don't want to see.
I like Cletus' suggestion of there being a prohibition against threadbanning a member once they've been invited to participate. I would add to that the requirement that if a member requested permission to join the discussion and permission was denied him or her, that a notice appear in the thread noting the request and the denial.