Clever! Perhaps in the spirit of 'never giving up', have the duration be permanent until expended, but can only have one such charge in effect at any given time. Also I think it would make more sense for the subject of the spell to speak the command word, so that they can more or less summon you to their aid when they need you most. This does remove some of the agency from the caster, though. Also is there any way for them to get back afterward?
Level: Sor/Wiz 9
Components: V, S, M, XP
Casting Time: 1 standard action
Range: Medium (100ft + 10ft/level)
Target: One creature / 3 levels
Duration: Concentration (maximum 1 minute/level)
Saving Throw: Will partial
Spell Resistance: Yes
Invoking the most powerful of your arcane energies, and sacrificing a bit of your soul in the process, you can cut off several creatures from their gods. Each creature targeted by the spell must make a Will save; failure indicates they are unable to cast divine spells or use supernatural or spell-like class gained through levels of classes which grant divine spellcasting, while success indicates they simply lose access to the two highest divine spell levels they can cast.
Material Component: All pieces of a broken holy symbol, worth at least 100gp.
XP Cost: 500xp per target
Next challenge: Another spell based on a song! (I might have to make a thread dedicated to this concept, actually.)