Praise be to Allah.
Yes, it is proven that the jinn can possess humans and harm them. Allaah says concerning those who deal in riba (interpretation of the meaning):
“Those who eat Riba will not stand (on the Day of Resurrection) except like the standing of a person beaten by Shaytaan (Satan) leading him to insanity”
i.e., when they rise from their graves at the Resurrection, they will only stand like one who has been possessed by the jinn because the belly of the one who deals in riba – Allaah forbid – will be huge, so when he wants to stand up he will fall down, and every time he wants to stand up he will fall down and be exposed before all people, so he is likened to the possessed man who has been overcome by the jinn and devils. This indicates that the jinn can possess humans, which is something that is proven and well known from visible experience. The jinn can enter humans and possess them, causing them to go insane, either because the human has harmed them and they want to take revenge on him, or because they are toying with him, or for some other purpose that they are seeking.
To sum up: it is proven that the jinn can harm humans by possessing them, and this is indicated by the Qur’aan and by visible experience that no one can deny but one who is ignorant.
The best means of dealing with that is reciting Qur’aan over the one who is affected, and treating him with ruqyah that contains du’aa’s narrated from the Prophet (peace and blessings of Allaah be upon him), and what he is suffering will disappear, in sha Allaah.
As for what some charlatans and liars do nowadays, such as prescribing some du’aa’s and drinks, claiming that this will bring the jinni out of the body of the person, the basic principle is that this is haraam and it is not permissible to treat such cases except with du’aa’s from Qur’aan and Sunnah. End quote.