If the purchase order has not been received (delivered quantity is zero) and not invoiced (invoiced quantity is zero), delete it by clicking the three-dot icon next to the order and clicking Delete Order.
If the order has been received but not invoiced, invoice it to complete it. Then either delete or reverse the resulting invoice from the Purchase Ledger.
If the order is invoiced but not received, select Receive Order or Close Lines to remove it from the Outstanding Orders list.