Gradebook

Hi Peter, I would probably do this with roles or with different courses.
If such users are taking different courses, then hide your Gradebook in Administration > Settings.
If they are taking the same course, then create another role, e.g. Participant (you may base it on the Student role) and set the capability titled "View the Grader Report" to Prevent. Assign learners to Participant role, and real students who are supposed to see grades to Student role.