Apple PowerBook (Duo 270C) - Manual (Page 216)

2. To deny access to a specific user, open the user icon.
To deny access to guests, open the icon.
3. If an X appears in the “Allow to connect” checkbox,
click to remove the X (to deny access).
4. Close the window and click Save in the dialog box.
If you deny access to a user who is currently connected, he or
she is disconnected immediately. That user is no longer a
member of any group, and can connect only as a guest.
If you deny access to guests, any guests currently connected
are disconnected immediately. Only registered users and
groups that you’ve specified will have access to your
computer.
200 Chapter 12: Using Your Computer on a Network