How do you restrict items being added to an existing purchase order.

When you 'Run Analysis' and check 'add to open orders' , All Orders will find the latest 'open' order for that vendor. Also if you have 'Use SO or PO location on linked POs' checked it will find the latest PO last 'open' PO with both the same vendor and location. The last 'open' order can be either 
1) If you have the preference to 'require order approval' checked it is the last order that meets the vendor or vendor/location test that is NOT approved. 
2) If you do require order approval it is the last order meets the vendor or vebdor/location criteria in which no items have yet to be received