First, remember that it is a part numbers "item class" that determines the default account/dept combination  to what account the purchased item will be charged.  Therefore, you can not just  "delete" a GL Dept on a purchase order as the account and dept fields require a "valid" GL Acct & Dept combination.
 
If you are using Evo, you can edit the department field and change the default account/department to a valid account/department combination as set up in the Chart of Accounts (COA).  You can not change to a new account/department code that has not already been previoulsy set up in the COA.  Any changes made to the account/dept during this maintence will affect new receipts only and not change previous receipts.
 
If you are not using Evo, the process becomes more complex as you will have to change the item class on the part number you wish to receive to the account/dept you want this charged to, reduce any open item remaining quantities on the purchase order, and then add the part number back to the PO with the remaing qty to be received and the updated item class account/department with be in effect for this part number added.
 
Don't know if this answers your question or raises more, but I hope it helps.
 
Dennis