The barbearian (sic, bear-totem) in my dnd group had a feat "mage slayer" from Complete Arcane which prevented mages from casting defensively. It worked sorta like this, except that we were interpreting it very wrong.
(...not surprising, for reasons.)
It's a bit weird, and doesn't really match what happens in OoTS. A mage threatened by the character (in this case an ogre with like 40 reach most times) automatically fails all concentration checks to cast defensively. Losing the spell but not provoking.
What we somehow missed was that a threatened mage is always aware of this, so there's no reason for them to try (they can still cast normally and hope to concentrate over the damage). Which makes it not match the OoTS feat. (We played it as they aren't aware, and then they even provoked on the failed attempt, which is doubly wrong)
Also when the Barbearian gained the ability to cast corrupt spells as a full-level caster, we forgot to penalize his caster level by 4 as per the feat. But, that was about when I quit in disgust anyway. That wasn't even the reason, but it didn't help...
In short I have no idea what the feat is. I kinda assumed it was homebrew, but Mr. Burlew seems pretty good about sticking to official material. Probably since so many people work hard on theorycrafting.
(I failed my spot check against ninjas)