If you want to inform everyone that you're chatting to someone else, you can use the
/all command - perhaps even create a personalised status or quicktext which you can assign a shortcut key to.
I know stuffplugNG has an ignore feature, which will discard any received messages from any contact. It doesn't notify them however, so as far as your contact is concerned you may be seen as a jerk for not replying to any of their messages.
Perhaps the tabbed feature may be useful to you when the new version or msgplus is released - you'll still get all your convos, but at least they will be tabbed nicely in one window