Workflow or Validation to prevent changes to fields

 I am trying to prevent users from changing the Industry picklist value if one of two read-only fields already contain a value. I am unable to use field validation on the field, and am attempting to create this functionality using before modified record saved to forbid users from changing the Industry.

As an example:

Industry cannot be changed when :

a) read-only Short Text field A has a value

OR

b) read-only Short Text field B has a value

Any person with Administrator role should be able to modify this value at any time.

I need to ensure that the Administrator is also able to update Short Text field A or Short Text field B, as some of my workflow attempts have prevented this.