Prism User Guide - Chapter 3. Transactions
3.7 Remove or Void Item

Remove Item
Select the item in the item list.
Click the Remove Item button.

Void Item
Users with the required security permission can void an item. Voiding an item keeps the item on the transaction but changes the item's quantity to zero. This can be useful for reporting and/or security reasons. Select the item in the item list and then click the Void button. To void an item, a user must belong to a group that is assigned the POS > Void Item permission. 
The "Print Zero Quantity Items" preference, when enabled, will include on the printed receipt any items that have a quantity of zero (e.g. voided items). If disabled, the printed receipt will not include items that have a quantity of zero.