I recall reading somewhere prohibitions against the use of magic in duels but can't find my much in the way of direct citations. Art of the Duel says that it's dishonorable for shugenja to use kiho or cast spells on themselves and prohibited for them to cast on bushi, and in "Victorious" Shiba Aikune is disqualified in the Emerald Championship for the Phoenix shugenja calling upon the kami during a duel, but this is much more about the Phoenix violating imperial sanction than interfering with a duel.
I was positive that spellcasting was prohibited in duels (not just dishonorable) but I can't find a direct reference. Can anyone help?