Dabbling in witchcraft or any of the occult practices CAN harm a person. The one behind these things is Satan and the demons. It is harmful spiritually, as well as can be harmful mentally and physically. There is an interesting account at Acts 16:16-18 which speaks of a young lady who practiced divination and was possessed by a demon.
Nope. Besides that silliness only works on people who believe in it BECAUSE they believe in it. So called spells and such have no power on anyine who ignores it. Its all smoke and mirrors, the right herbs and poisons and a whole lot of self-fulfilling prophesy.