You know guys, instead of all the radioactive or poisonous elements you guys are suggesting, she could just transmute some of his non-vital viscera into a contained mass of nitroglycerin and tell him that if he moves too suddenly he'll explode (with a prior demonstration to get the idea into his head). Now he'll be drowning in terror for as long as he can manage to not disturb the ultra-volatile bomb within him before he inevitably trips it against his will.