Thanks, you guys.
Didn't have as much patience as I thought with the hot water, so I just went straight for the Xacto.
Snow-glo was in demon mode and decided not only that she wouldn't give me her head, but also that she wouldn't let go of her hair. She had lumps of glue in there that were so huge, they couldn't even fit through the neck hole!
Rainbow Flash, however, was in angel mode: quick and easy head removal and even quicker hair removal. That's my girl.