The process to lock the fields is the easiest method.
You may be able to setup users with only the Status Report - Create All right, and have a process that removes the Status Report - Edit instance right (via a Gel script) when the Status changes to final. Check with your CA Clarity PPM developer to be sure. That way, an administrator with the Status Report - Edit All right can still edit them, and the instance right can be granted if the user made the change accidentally.