If someone fears that he may have affected someone else with the evil eye because he liked or admired him, he should say say: Barak Allahu laka or ‘alayk

For protection from the jinn, you can say these du`as: 1- Seek refuge with Allah from the jinn: 2 – Recite al-Mu’awwadhatayn 3 – Recite Ayat al-Kursi 4 – Recite Surat al-Baqarah 5 – Recite the last verses of Surat al-Baqarah 6 – Recite “La ilaha ill-Allah wahdahu la sharika lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shayin qadir 7 – Remember Allah often (dhikr) 8 – Say the adhan (call to prayer) 9 – Recite the Quran as it offers protection against the shaytan

Jinn possession is proven by the consensus of the imams of Ahl al-Sunnah wal-Jama`ah. If a person who has this sickness is rational, then he has the choice and he will be brought to account for his words and actions. But if this sickness has overwhelmed him to the point that he has lost his mind and free will, then he is like one who is insane and is not accountable.

The jinn are part of the creation of Allah whose existence we must believe in, because Allah mentions them in many verses of the Quran, and there is even a Surah named after them, namely Surah al-Jinn. Their existence is a matter upon which there is consensus. For more, see the detailed answer.