From that it seems simple enough, just not entirely logical. A lot of people would probably settle for just having it disappear from the user's inbox. I mean, I doubt it's about them having the information from the PM, because then they could just as easily paste that in a text file. I think it's more to keep them from getting *future* entries to said PM. It would obviously make the most sense if they could drop in AND out, and only have the messages they were given permission to, but, that would require a change on XenForo's part itself, I'd imagine.