Is there any way to restrict users in selecting wrong Natural Account in Expense PO Distribution?

In case of expense PO's, user is required to select the valid Department Code in COA and sometimes by mistake they are updating natural account value to Asset/Liability which is not supposed to happen.

We can think of using security rules restricting other than expense account to PO User responsibility but does it have any impact on other PO's where in we have Inventory Items (which have other than expense accounts as account gl code)

Any pointers on this?

Regards

Uday