The rule with Partially Processed purchase orders and sales orders is that they may not be 'Cancelled'.
In the case of purchase orders, the logic is that the PO has generated a GRN and therefore the order was not 'Cancelled'.
To remove a partially processed purchase order from the list of 'In Progress' orders, do the following:
Select the relevant order from the list. Double-click to open.
On the tool bar, click the 
Set Partial Order to Fully Processed button.
The status of the purchase order will change from Partially Processed to Processed.