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Hiding tiers and marking sold out
Any tier can be hidden or marked sold out at any time, including mid-sale. Hidden tiers vanish from the event page; sold-out ones show but can't be bought.
Neither affects tickets already sold. Somebody holding one still gets in.
Stopping sales without deleting
To stop a tier selling permanently, set its allocation to the number already sold. It reads as sold out and nobody loses anything.
Deleting a tier that has sold tickets is refused, for the obvious reason.