Have you considered using a Drop Ship type PO?  Those should complete when fully invoiced (with no RoG).

Edit: And a $0 invoice should work too, if you aren't receiving a proper invoice.

Completing a PO has specific technical requirements that are controlled by the program, so there is no easy way to force a PO closed like you are suggesting.

Thank you Kevin for the suggestion.  We never invoice these items.  We are a truck upfitter, we order a chassis based on our customers request using a PO, we are not a truck dealer and do not have a dealer license.  When the chassis comes in we install the tanks and all the equipment and return the completed unit back to the truck dealer.  We invoice the truck dealer for our work.  When we close out the Work TIcket and Invoice the work, the PO for the Chassis stays open.  We are on 100 premium (SQL).  I have a little script set up to update the sql record.  It would be easier to allow the person billing do it from the PO Screen.   My thought is to create a checkbox on the menu that executes a VB script that will run the SQL script.  Again, easier if I can just add to the existing drop down box.   

That is never going to be possible unless you get a Master Developer to make an enhancement.

Alternative idea: turn on PO history with the option enabled to Retain set to Prompt... then delete the PO.  You can then set the cancellation code appropriately during the deletion (Cancelled, Received...).  Keeps your open PO table clean, with completed PO's in the history table.

Direct SQL edits are not recommended.  (I limit myself to SQL edits of UDF / UDT data only to avoid any unforeseen issues in bypassing Sage programming for normal fields).