How to prevent users from overriding default expense account

Summary:

How do we prevent user from updating default expense account (defaulted based on category) on GL string?

Content (please ensure you mask any confidential information):

We have need to prevent user from updating default expense account assigned based on category. User can override any other segment but should not be allowed to update account.

Version (include the version you are using, if applicable):

Oracle Fusion Expense Patch 26A

Code Snippet (add any code snippets that support your topic, if applicable):