It's been a really long time since I read that book, so I can't tell whether this should be built as a limitation into the spell or as a defense that spellcasters have. I mean, that spell didn't target anyone directly, so why does the wizard get a pass? I'm leaning toward the latter interpretation, much as Lucius has built, but I don't think I'd build it as EDM or Desolid. I'd be more inclined to go with conditional defenses or some sort of perpetual Suppress.