Depends what you mean by 'assigned'.
- If you mean that your users can see the course on the front page, then I'm guessing you've got either categories view or combination view being used, both of which show all availabe courses, not just the ones that the user is enrolled in.
- If you mean that your stuents are enrolled in the course, then check that you haven't added any site-wide student roles (done in the Site Admin block under Users/Permissions/Assign_System_Roles) - if you do this then any student in this role will be given student rights to all courses in your site.
Hope this helps,
Mark.