Your best bet would be to call over the tournament arbiter and get your opponent disqualified, assuming he is unwilling to take the move back.
Ng3

can someone tell me a move that can be used against Ng3
are you speaking of an "opening" move 1. N-g3...because it doesn't exist. the knight can't go to g3 on its first move. maybe you actually are referring to the infamous Amar Opening , 1. N-h3 (which incidentally i've had a fair amount of success with) as TheGrobe states, you must be more specific.
can someone tell me a move that can be used against Ng3