**Do NOT delete the item, as that will cause you to lose any previous rental history**
First, you'll want to edit the item with an indicator such as "DO NOT RENT" or "DO NOT USE", so that it is clear it is not to be added to future projects.
Next, you'll adjust that item's quantity to "0", but ensure that you still have "Track Quantity" enabled. This will ensure that a conflict appears should the item ever be added to a project.
You've now made it clear that this item should not be added to an order and kept its history in your account.