Are there people whom God will not forgive if they seek forgiveness, and is the story true about the grave robber whom the Prophet (peace be upon him) did not answer when he wanted to repent?

Allah Almighty forgives all sins through repentance, and one who repents from a sin is like one who has no sin. However, repentance has conditions. As for the story mentioned, it is not found in the sources, and it is not conceivable that such an act would have been committed by the Companions.