I need to create a formula that locks opportunities once they reach a certain stage. We want only a the administrator to be able to make changes once the opportunity hits the stage so we can make sure the date and the amounts don't change. Can someone help explain how I would go about creating the formula or if this is even possible?

I would like to take a custom check box field "requirements spec sheet reviewed by engineer" only modifyable by certain users.  I cannot seem to find any literature on creating a validation rule where i can list out specified users that should have access to modifying that field and the salesforce general examples dont even pass a syntax check.

My goal for this is that i dont want the field to be able to be selected by any user as it is a gatekeeper field.  I only want administrators and engineers able to check off this box. 

Is this possible the way i am looking to do it?

Thanks!