One of the sample spells in YS requires you to tag a "tree aspect" in order to entangle people and prevent movement. So without that narrative aspect you can't cast the spell.
This spell requires a spell casting aspect. If we want to get technical, we can make it the same as above and say that the person has to make an assessment on the target to tag an aspect that relates to their casting ability.
Filling the 3rd stress box doesn't prevent spell casting, it hampers spell casting. Now instead of using a box, the caster has to use a consequence. It's requiring more and more effort on the spellcasters part to cast each spell. He has less boxes to choose from, that hampers casting.
I also don't think I'm making sketchy claims. By that token were-wolves running around is sketchy. It's a game about the supernatural where mundane things have extraordinary connetations.
I have a friend who suffers migranes. They affect her mood, her personality, make her tired and unable to do the typical things most people do, like go out on a sunny day. That affects a person on an emotional level. She's tried medications and a lot of other things but the only thing she says works is her weekly visit to have someone do reike.
Take it for whatever you want it to be. Maybe it's a placebo, but in a game where magic exists, things like that are magic.
EDIT: I left the computer and thought about it some more.
IF you require an aspect to tag (by declaring/guessing/assessing) - an aspect indicating that your target is a spell caster - then it limits how much you can attack doing mental stress. The first one would be a free tag, but every attack after that would require a FP.
It certainly makes it a lot less OP, if that's your concern.
