If your list of non-respondents for a Feedback activity shows all users on your site who have not responded, rather than just the non-responders from your course, then the 'Authenticated User' role on your site probably allows the 'complete a feedback' setting.
To fix this:
- go to Administration> Site administration> Users> Permissions> Define roles
- in the 'filter' box (under the 3 big boxes), type in 'Complete a feedback' - (Tip: DON'T hit the 'Enter' key)
- change the setting to 'not set'
- click on the 'save changes' button