If a person undoes spells or witchcraft, one of two scenarios must apply: 

1-

He uses shar’i ruqyahs and ta’awwudhaat (prayers seeking refuge with Allaah) that were narrated from the Prophet (peace and blessings of Allaah be upon him), and permissible remedies. There is nothing wrong with this, rather it is mustahabb. 

2-

He treats the spell by doing witchcraft which means drawing close to the jinn by offering sacrifices or doing other acts of worship. This is not permissible, because it is the handiwork of the shaytaan and is in fact major shirk, so we must beware of it. It is also not permissible to deal with it by asking sorcerers, fortune-tellers and charlatans or doing what they say. The Messenger (peace and blessings of Allaah be upon him) has warned against going to them, asking them questions and believing them. He (peace and blessings of Allaah be upon him) said: “Whoever goes to a fortune-teller and asks him about something, his prayer will not be accepted for forty days.” Narrated by Muslim (4137). 

And the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever goes to a soothsayer or fortune-teller and believes what he says has disbelieved in that which was revealed to Muhammad (peace and blessings of Allaah be upon him).” Narrated by Ahmad in al-Musnad (9171) and classed as saheeh by al-Albaani in Saheeh al-Jaami’, hadeeth no. 5939. 

So it is not permissible to go to practitioners of witchcraft or ask them anything or believe them. 

Abu Dawood narrated in his Sunan (3370) that Jaabir ibn ‘Abd-Allaah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) was asked about al-nushrah and he said: “It is the handiwork of the shaytaan.” Classed as saheeh by al-Albaani. 

Ibn al-Qayyim said in I’laam al-Muwaqqi’een (4/399): Nushrah means undoing witchcraft. It is of two types: undoing witchcraft by means of witchcraft, which is the handiwork of the shaytaan, because witchcraft is the handiwork of the shaytaan, and the one who engages in nushrah and the one for whom it is done draw close to him by means of what he likes, thus undoing the spell. The second type is undoing witchcraft by means of ruqyah, ta’awwudhaat, du’aa’s and permissible kinds of medicine. This is permissible, and is in fact mustahabb. End quote. 

Thus you will know that if this imam is of the type who undoes witchcraft by means of haraam methods, then going to him is a sin, and may constitute kufr. What the one who has gone to him must do is repent from that, and Allaah accepts the repentance of the one who repents. If he did this out of ignorance then we hope that there is no sin on him, but he must give that up and resolve not to do it again in the future. 

If he is one of the type who treat those who are bewitched by means of the Holy Qur’aan and du’aa’s narrated from the Prophet (peace and blessings of Allaah be upon him), then there is nothing wrong with going to him, and that is not a sin. 

And Allaah knows best.