I figure that there has to be some sort of mechanic for that. Say the DC for casting off-list is, I dunno +10. So a warcaster (most generalist hedgewizards) trying to cast, say, undetectable alignment has to hit a DC 26.
Say a level 2: +2 ranks, +3 class, +4 stat needs a 17 on d20. That's too easy actually.
And just for the record all the big skill booster spells (wieldskill, guidance of the avatar, etc) get noped.