Convert a tracked inventory item to untracked when you've used it in a transaction, or made an adjustment to it
Xero won't let you uncheck the checkbox 'I track this item', but you can manage this in another way.
A process you can follow is:
Add a new untracked inventory item. The item's code must be unique, but all other item details can be identical to your tracked inventory item.
Enter a 'decrease quantity' inventory adjustment to your tracked inventory item to reduce its quantity on hand to 0. Xero will reduce the item's value on hand to 0 as well.
Change your tracked item's Item Name to wording which will be an alert to the user, such as 'Do not use'.
Use the untracked item you created for all your transactions from now on.