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There are many mantras to nullify the effects of black magic.
1) There is a homam on Ucchishta Mahaaganapati that can be most helpful in this regard. contact reputed upaasakaas for this purpose. One person I know in this regard is Pandit. DAKSHINAMOORTHI avargal about whom I often I refer in this forum. He is a highly saatwik but courageous upaasakaa, and very philosophical in his approach. As he is doing Ganapathi homam on all chaturthis, you can get in touch with him in this regard if you are interested. If you are in chennai, you can contact Guruji directly at his residence in Babanagar, Villivakkam.
You may approach any person whom you deem to be a spiritual expert also in this regard (But please note that Ucchishta Ganapathi recitals are not done by everyone unlike Mahaa Ganapathi japam that is known to all vedic shastris. That is why I referred Pandit avargal's name).
2) Chanting the following slokha :Paraabhichaara samanah Dukha Panjana kaarakah
Lavah Druti kalaa KAshtA Nimaeshah kati muhUrthahah" from Ganapathy Sahasranaamam is a powerful remedy for black magic. You have to keep Holy Ash in your right hand and chant the above mantra 108 times and then smear your forehead (or on the forehead of the person who is suspected to be afflicted with black magic) after the recital. Within 48 days you will find good improvement.
3) Chanting/ listening to Sarabha Upanishad can be a good healer for this.
4) Worship of Goddess Pratiyangira helps in removing black magic.
5) Reciting Hanumaan Chaalisaa 9 times daily for 18 days gives good recovery from evil effects of black magic.
6) Performance of Dattatreya homam on Pournami days will help.
You may perform any one or more of the above as per your convenience.