Allah The Glorious did create for humans and Jinns two different worlds. Human world is for us and Jinn world for the Jinns. No Jinn can be a human and no human can be a Jinn.
In Quran, we read about those humans who wanted help from the Jinns but got more hardships from the Jinns. (وأنه كان رجال من الإنس يعوذون برجال من الجن فزادوهم رهقا )(Sura Al-Jinn: 6).
Jinns have no control over human beings in usual cases, but there are some wrong acts which make the human being so weak that some Jinn can harm him.
Many disliked acts are mentioned in Hadeeths can cause such weakness. Sleeping in wrong time e.g. between Asr and Maghrib, or urinating inside water, eating Najis food etc.
Jinn can not posses a human being in usual cases. Allah (SWT) did not leave us unprotected from the harm of bad Jinns.
Many people think that or others are under the posses by Jinns, but it is not the reality all the times. It can be a mental or psychological case which they think it from the Jinns. So, we can not believe any one who thinks in this way blaming Jinns.
Even those who due to bad acts became very weak in front of Jinns, can easily go out of their control by reciting Quran and Du'a. Reciting Ayatul Kursi and Sura Falaq and Sura Naas has great benefits in protection, prevention and cure. Also reciting Verse 35 in Sura Ar- Rahman in clear or loud voice is very useful in removing the bad effect of Jinns and driving them away.
Repeating the names of Allah (SWT) and Salawaat and Istighfaar are always best protection from any harm.