Is your group a Meet-up group? I'd worry about the ability to pre-screen well there. I use to lead a backpacking trips for the local Sierra Club, and I think one of a leader's responsibilities is definitely to not let people attend if they are not properly prepared, so I'm with you on not letting the guy attend. Of course, I found that kind of hard to do myself because I'm a nice guy and I don't like rejecting people. Still, it's for their own good and the good of everyone else on the trip.
I never had problems with people who were as lame and stubborn as that guy sounds, but once, on a beginners backpack I was leading, I was looking at this woman with her pack on, thinking "hmmm, what's wrong with this picture?", when I realized she had her backpack on upside down. I also had a participant once who I soon realized was truly, seriously mentally ill, as well as profoundly annoying. Not good