If you have an POP Party that is open, but you want to remove the items that have been claimed, go to your Orders page, select the items you want to remove from the Party, scroll down and click Remove Selected from Parties. It will give you the option of which Party to remove from. Select and confirm.
If you have an item set to load to a Party, but it sells elsewhere before the Party begins, simply go to your Inventory page. Find the item, select and highlight it, then mark it as Sold Outside of POP. This will move the item to your Orders page. From there, select the item, scroll down and click "Remove from Parties" and select the Party it was set to load to.