If all else fails and the non kamae guy herein keeps standing in a manner like a judoka or iaido practioner a well directed gentle kick to his kneecap will soon make him shuffle into the correct stance.It may sound a bit non aiki, but it works for me.
You are kinder that I am! I typically will execute a fast front snap kick that stops within an inch of the groin area. I then talk about proper kamae as being a way to control both distance and the nature of the attack....
At another level, it is simply disrespectful for a student to do something other than what the teacher directs the students to do.