In either Classic SSP or RSSP is it possible to restrict Enter Requisition Line Source Type

Summary:

In our organization, we auto approve requisitions under a certain dollar amount. We have users who are entering a low value dollar amount of a supplier's item because they don't know the actual cost and bypassing the approval that way. We're wondering if we can prevent users from entering "Source Type: Supplier" lines in Enter Requisition Line so that they can only order from the Catalog and not add lines for non-inventory items. We only want them to be able to Enter Requisition lines for "Source Type: Inventory" and get their supplier items from the Catalogs with BPAs attached.