This functionality is new as of version 4.0.
To Cancel All Remaining Unreceived Items:
Navigate to the 'Purchase Order Details' screen → 'Purchase Order' tab:
Click the 'Cancel Remaining Unreceived Items' button.
Confirm the action.
The 'Cancelled Qty' for the unreceived Purchase Order Items will be updated with the difference between the Ordered Quantity and the Received Quantity.
The 'Status' of the Purchase Order will be updated:
If some of the Purchase Order Items were Received, the Status will be updated to 'Received'.
If all of the Purchase Order Items were Cancelled, the Status will be updated to 'Cancelled'.