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 save! This will now add that item to your discontinued items list and remove it from the kiosk store.