I don't see the point of this, if you want to you could send someone a private messege (PM) or you could invite them to chat through steam or xfire and talk about whatever you don't want other people seeing.
Trasgu is a general forum member. Should we make a private shout and then exclude him from it? lol. Besides, there are many tabs on the top of the shoutbox for members, mods, super mods and admin.
If you want a chat more private then that, email or IM. Or Steam.