Spell Intercept.

The logic behind it was simple, the caster would think of a magic command code, so by letting the caster’s mind become confused, the caster’s control of magic would be obstructed. It was like a person counting down in his mind, if someone spouted some random numbers, he would be affected.

Index couldn’t use magic.

But she could try to let the opposing magician self-destruct.

Toaru Majutsu no Index vol.5