The Zanpakutō of a Shinigami can only cleanse spirits of the sins they committed as Hollows. If the spirit committed heinous crimes as a Human, it will not be sent to Soul Society, but through the Gates of Hell.
When Ichigo Kurosaki defeated the Hollow Shrieker, he did so by slashing Shrieker's Hollow mask. The Gates appear to be formed from energy or light which emerges from the wound. The Gates of Hell are chained shut and adorned with the bandaged head and torso of a skeleton on each door. The arms of both skeletons are positioned at an angle to pull the gates open when it is summoned.
On the inside, the Gates appear to be prevented from opening fully by two thick ropes with charms attached to them. The face and left arm of a large demonic creature can be seen behind the Gates. It has armor plating on its upper arm and it wields a blade. Its left arm and face are covered in tattoos resembling tribal markings. The demon skewers the Hollow entering Hell with its blade, and pulls it through the Gates, while laughing. The Gates then close and the entire structure cracks and crumbles into nothing.