Limit account codes employee's can select to post expenses to

Summary:

We want to know the best way to limit what accounts employees can code expenses to. The expense type sets the account but the account flex field can still be updated before submission by the user. This field allows employees to enter any valid account on our GL.

Ideally we would be able to limit users to coding to certain account categories, such as the category for us that contains all of our departmental expenses. Currently an employee can post a Procurement Card/Reimbursement transaction to retained earnings, cash, or revenue etc. What is the best way to implement an internal control? Is there a way to lock the account flex field to the expense type?