Rather than delete an item, you can Discontinue it.
This removes the item from the store display and will not affect invoices that already contain that item.
On the admin side of Triib, simply click Store > View Store and click on the product you wish to remove from the store.
Scroll down until you see the Discontinued box, check that and hit save! This will now add that item to your discontinued items list and remove it from the Kiosk Store.