We've had quite the discussion in the comments section, over the workplace 'witch'.
Says Carlos:Well, if she says that she is a witch, then she is most likely a Wiccan, which is a pagan religion whose participants call themselves witches. I wouldn't insult her faith because that may get you in trouble at work yourself. I think "spells" are a form of prayer for that religion.
I think you should be charitable in how you deal with her, but make it clear that you're not interested (assuming you're not). You should pray for her as well.
I believe I also said we shouldn't be sarcastic and that we should pray for her. But the discussion continues:
I agree with Carlos. "Witch" is not just a fantasy creature like unicorns. Wiccans, and many Pagans in general, use the terms "witch" and "warlock" to describe their role in their faith. I can tell you from personal experience that calling her silly and telling her you'll pray for her is the number one way to guarantee she will never EVER listen to anything a Christian has to say ever again and will, in fact, make sure all of her friends know to hate Christians as well. Again, personal experience. I know a number of pagan people who decided to invest firmly in that faith because of the final straw when the Christians treated them with mocking and disrespect.
One main idea that many pagans (Wiccans especially) hold as a foundation is "An harm none". If you need to talk to this co-worker in her language, you could point out that doing something to you ("casting spells") against your will is harmful. Ignore the fact that we don't believe in spells... she obviously does. She needs a reason to stop that makes sense to *her*. The best way to get through to her may be you taking the time to figure out some of her faith language.
Wiccans brought me back to Christ after a traumatic absense. Most of them have a firmer and stronger faith than many Christians I know; it's just faith in the wrong thing. I really hate to see when a loving group of people such as these are attacked or demeaned by those who don't take the time to understand.