Locking fields for Excel import

Hi all

I have a how to question regarding how to lock fields so it is not possible to update them via the Excel online import feature. I tried locking the fields on entity level by using creating a business rule. But when I Export to Excel online it is still possible to change the values of the locked fields and reimport the changes. It seems like a design flaw that this is actually possible.

Any suggestions on how to lock the fields completely?