As I have mentioned, the Wiccan Rede states: "An ye harm none, do what you will." A witch may do as s/he pleases as long as it harms no one and impedes no one's free will.
There are also a couple of by-laws. The Three-fold Law states that whatever you do -- good or bad -- will come back to you three times over. This is much like the Golden Rule, but I think it paints a better picture of exactly what will happen to trouble-makers.
Another Wiccan belief is the Law of Responsibility. This says that even if we accidentally impede another's free will, we must take steps to better the situation. Hope that you never need to follow this law!