In Bleach, characters do battle with spirits and supernatural energy. Often, this takes the form of black-robed Soul Reapers hunting and slaying monstrous Hollows, which are the eternally hungry and tormented souls of the undead. A Soul Reaper can handle the job with their Zanpakuto, their soul-cutting sword, but some Hollows might be difficult to handle.
Most Hollows are ordinary and fairly weak, with each one being made up of a single person's soul. Even unranked Soul Reapers can handle them, but sometimes, hundreds of cannibal souls may merge into a Menos Grande, and this is a serious headache for any Soul Reaper. Is it even possible to purify these things?
How Soul Purification Works
Only Soul Reapers can escort the souls of the dead to Soul Society or deliver them to Hell, as their name suggests. By contrast, Quincy archers, the "monks of destruction," destroy Hollows entirely with their arrows, whereas a Soul Reaper's Zanpakuto hekps them pass on. Billions of souls exist between Soul Society and Earth in the early 21st century, and those souls cycle between the two worlds, being born and reborn to maintain balance. It's the Soul Reapers who maintain that balance, and that means escorting Plus souls to the afterlife and dealing with Hollows.
When a person dies, their soul will either become a Plus, or mutate into a Hollow over time. A Plus is a regular ghost, and a Soul Reaper can send that Plus on its way to Soul Society with the simple Konso technique. Meanwhile, a soul becomes a jibakurei if they're bound to the Earth with negative emotions. Eventually, their Chain of Fate opens a hole in their chest, warping the soul into a Hollow, a beast that consumes souls — and only a Soul Reaper can save it. The Soul Reaper will slay the Hollow with their Zanpakuto, then perform the sublimation technique to cleanse the Hollow of its misdeeds, making it worthy of Soul Society. If the Hollow committed truly horrendous acts in life and/or as a Hollow, though, it will be sent to Hell itself. Either way, the Hollow has been dealt with.
But what if a Hollow isn't just one soul?
The Problem of Menos Grande Hollows
Some Hollows turn cannibal, and if enough cannibal Hollows meld together out of mutual hunger, they form a Gillian-class Menos Grande, a titan that operates on sheer instinct, with a hive mind of hundreds of Hollows inside. Now, what a Soul Reaper can do when faced with such a beast isn't entirely clear, lore-wise. Powerful Soul Reapers, like a Lieutenant or a Captain, can handle a Gillian in battle and slay it, but that doesn't answer what happens to the souls inside. One possibility is that a sufficiently powerful Soul Reaper could perform a large-scale sublimation technique, and purify all of those souls at once, sending them to Soul Society as a group. Or, the Soul Reaper might take the time to evaluate each component soul individually before sending them where they belong. But this isn't ever seen in the series.
Then there's the matter of Menos who are no longer hive-minded. It's possible for one strong-willed Hollow inside a Gillian to take command of the entire creature, and if this happens, that Hollow will mutate the Gillian's body to fit their image, and the Hollow regains its individuality. Such Hollows must continue to hunt other Hollows, often in the Hueco Mundo desert, to maintain the strength necessary to keep this form. Such Menos can power up and evolve into a mighty Adjuchas class, and a select few have the potential to become a Vasto Lorde, a Hollow brimming with thousands, or possibly millions, of souls inside. But the common factor is that one soul is in charge.
The Espada and all the Arrancars under Aizen's command are like this, Menos who have become unmasked and taken on human forms. Most of them ended up slain at the hands of the Soul Reapers, but it's not clear what happened to them next. When Captain Byakuya Kuchiki slew Zommari Rureaux, the 7th Espada, did Zommari's component souls scatter to the four winds? Byakuya didn't perform any rituals, so perhaps Zommari's souls (including the dominant one) were simply destroyed instead. After all, Zommari was extremely fearful of death, and he melted away into nothing when Byakuya struck him down.
Then again, it's possible that such powerful souls went to Soul Society automatically. Readers might never know for sure, because at that point, the story didn't concern itself at all with matters of soul purification and justice. Only the early chapters and episodes involved actual soul reaping.