Is it possible to hide buyer names in dropdowns?

We are trying to see if we can hide a specific group of names in the Buyer Dropdown field in Manage Orders.

Currently we have two roles PO Buyer and PO Inquiry that have been set up in the Procurements Agent table. (Manage Purchase Orders). The buyer has full access while staff assigned PO Inquiry has View only.

What we noticed is that the people with PO Inquiry, is that their names appear in the Buyer Dropdowns.

This was not problem but recently we have discovered that if some selects a non buyer from the dropdown they get full access to the PO and can make CO and changes at the PO level. In short they become the Buyer for that PO.