Need to have limited no. of people to be allowed as a Requestor on the Purchase Orders

Summary:

Content (required): Currently Oracle allows any employee to be selected as a 'Requestor' on the PO line and there is no role or access required for an employee to be a Requestor on the PO line . 

There is a critical business need to have limited no. of People to be allowed as a Requestor on the Purchase Orders so please let me know if there's any way to achieve that . Thanks.

Version (include the version you are using, if applicable): Currently Oracle allows any employee to be selected as a 'Requestor' on the PO line and there is no role or access required for an employee to be a Requestor on the PO line .