"If magic could simply be taken, people like Voldemort or Grindelwald, or any madman, be they magic or mundane, would have had no problems bending the world to their will. You can put a trace on magic, you can follow its signature, but you cannot force it gone. It is intrinsic to magical beings. It would be like... removing a brain and expecting the body to still function. It cannot be taken. But it can be given, under specific circumstances."