Council - 42:24
"Do they say, "Has he fabricated a lie about Allah"? If Allah wills, He can seal your heart. Allah erases falsehood and establishes truth in His words. He knows the most secret thoughts."
To seal the heart; to remove the understanding completely, to make it unable to find the right path ever again:
The Women/4:155
"Allah sealed their understanding because of their breaking their word, because they disbelieved in the signs and proofs of Allah, because they killed the prophets unjustly, and because they said: "We are fixed-minded", yes, because of their disbelief. Therefore few of them believe."
The Kneeling/45:23
Have you paid attention to him who takes his desire as a god? Allah has deliberately led him astray, and has sealed his hearing and his brain and put a veil over his sight. Who can guide him to the truth except Allah? Do you not take admonition?
The Hypocrites/63:3
Because they disbelieved after they believed, so their hearts were sealed; they do not understand.
The Pilgrimage/22:46
Have they not traveled the earth with thinking minds and hearing ears? The real blindness is not the blindness of the eyes, but the blindness of the hearts in the breasts.
The Heights/7:100
Has it not become evident to those who inherited the earth in place of the previous generations that if We willed, We could punish them with their sins and seal their hearts so that they could not hear?