it sure can hurt ace since we have seen how amaterasu devours fire.
it may be able to hurt ao kiji
it may be able to hurt caribou
aaaand that's it. the rest of the users are virtually immune to it.
Yeah I don't see why they couldn't just remove the part that was affected by Amaterasu. Like how Akainu has his magma fists, effectively he's just firing off his fist as an attack. They should easily just be able to purge of the part afflicted by Amaterasu.