The 310th prohibition is that we are forbidden from allowing a sorceress to live.
The source of this prohibition is G‑d's statement,1 "Do not allow a sorceress to live."
If we would exempt a sorceress from the death penalty, we would transgress a prohibition. This is unlike exempting from execution another type of transgressor, which would constitute only the nullification of a positive commandment. (This applies equally to a sorcerer. Note, the death penalty is because idolatrous practices were employed, not because there was "a pact with the Devil" -J.S.)