Updated: Aug 24, 2020
The Prophet (ﷺ) was asked about nashrah and he said, "That is the work of the devil." [Abu Dawud; Sahih]
Nashrah, in this hadith, is the act of undoing or treating a sihr (magic, spell) with sihr.
The Prophet describing it as "the work of the devil" is more emphatic than saying it is haram, because it implies that it is wholly despicable, ugly and repulsive.
It is not permissible to treat sihr with sihr, rather sihr is to be treated by means of recitations, duas and other permissible modes of treatment.
As for sihr, it is kufr (disbelief) and apostasy, and puts one beyond the pale of Islam. It is not permissible to engage in it or to go to a practitioner of witchcraft seeking a cure.