It seems like there's a heirarchy of supernatural powers on the show; you can only cast a spell on someone if you're more powerful than they are. I think it was in 1795 when Joshua called in some witch woman to try to release Barnabas from his curse, but she wasn't as powerful as Angelique, so she ended up burning in flame. Only Nicolas Blair seemed to be able to cast spells on her, at least that I can remember.