From the online help:
Student Access | Top
You can deny and allow students access to your course at any time. This feature is particularly useful when you are making changes to your course and want to temporarily disallow students from accessing it.
The following procedure denies all students access to your course. If you want to deny access to a group of students, list a subset of students first. You can also deny access to one student at a time by editing the student's record. Note: Denying access to students does not delete any student information. To permanently remove students from your course, you must delete their records. Also, you cannot deny access to inactive users.
You may want to remove students with denied access from the Student table. Note: Depending on administrator settings, this option may not be available.
To hide access denied users:
Go to the Control Panel, Manage Course, Manage Students. Click the student’s name in the Gradebook (Manage Students). Click the Deny Access button under the student’s name.