Sexual fantasy during intimate relationship, how do I get rid of it? And is it prohibited?

All praise be to Allah and peace be upon the Messenger of Allah, upon his family, his Companions, and those who follow him.

How to get rid of that? There are some matters which a Muslim should do to get rid of all sins and faults.

1- Practicing piety, revering and fearing of Allah (Glory be to Him) which is the greatest barrier which stops a person from falling into minor and major sins. When a person knows that Allah (Glory be to Him) is acquainted with his actions and secrets, a person fears Allah (Glory be to Him).

Fear is the solid barrier before violent air streams and few people who stand before the pushes of air as Sayyid Qutb said.

2- Sincere repentance includes regret and giving up sins out of fear from Allah (Glory be to Him) and sincere determination not to commit it again; so the greatest blessing given to a person is that Allah guides a person to repent and regret what he has done.

3- Increasing pious acts in the true sense.

No doubt, if man draws near to Allah, recites the Qur'an prudently, and directs his heart only to him, the heart shall accept the truth, get affected by recitation and admonition, and stop wishing the unlawful.

4- Caution against impenitence for feat that a barrier may be set between your heart and the acceptance of the truth as Allah (may He be Exalted) says: "Nay! But on their hearts is the Rân (covering of sins and evil deeds) which they used to earn." [Surat Al Mutaffifin: 14].

5- To feel that the person who does sins and faults shall incur the wrath of Allah which will put him under His Punishment.

6- To fill your times with obligatory acts, such as: The five daily Salahs, obligatory fasting, filial obedience, establishing the ties of kinship, and other obligatory duties. Furthermore, keeping to Sunan and desirable acts, such as daily Sunan, observing voluntary night Salah, desirable charities, praising Allah, remembering death and its harshness, the grave and its darkness, and the Day of Recompense and its horrors. Allah (may He be Exalted) says: "O mankind! Fear your Lord and be dutiful to Him! Verily, the earthquake of the Hour (of Judgment) is a terrible thing. * The Day you shall see it, every nursing mother will forget her nursling, and every pregnant one will drop her load, and you shall see mankind as in a drunken state, yet they will not be drunken, but severe will be the Torment of Allâh." [Surat Hajj: 1 - 2].

And Allah knows best!