What is the ruling on one who takes pleasure in sin, despite knowing of its prohibition and that Allah sees him, and then completes it?

Committing a sin for immediate pleasure, and finding delight in it, does not negate the root of faith, as long as one acknowledges the prohibition of what they are doing. Furthermore, their open defiance of their Lord in committing it does not exclude them from the community of Muslims. If they do not repent, they are subject to the will of Allah the Almighty: He may punish them if He wills, or He may forgive them if He wills.