For me as a GM it is all I need. When I feel like a Shugenja is overusing his/her power as a caster I simply ask: When was the last time you prayed without casting? When did you last visit a shrine or temple? When was the last time you offered something to the kami?
If the player can't answer these: Sorry, but you no longer feel the support of the kami. They don't answer your prayer, since you forgot your priestly duties.
Completely sincere question: how often have you had this interaction while GMing? How did the player react? How did it work out?