Thankyou Richard Oelmann for your response.
If capability enrol/manual:unenrolself is removed, then there will be no option for student to unenroll from course. But requirement is to make that
1) Student can request for unenrollment from the course.
2)Admin can approve or reject the request.
Right now moodle capability is
1)If student is permitted to unenroll from course ,then student can directly unenrolled from course without admin approval