Go to inventory, then manage, and select what you want to delete. You may click the "select all" option to select all inventory.

Scroll all the way down and you will see the "Delete" button.  Click here and it will ask you to delete, or simply set to 0.  Once you have confirmed deletion, this will remove your items completely from POP.  

Be aware that once your items have been removed, there is no reversing this action.