Network users can gain access to the Extensions Manager control panel by holding down the space bar when restarting the computer. However, they cannot disable the Multiple Users environment from within the Extensions Manager unless they are the administrator.
If you do not wish for users other than the administrator to gain access to the Extensions Manager control panel in this way, you should manually disable the "EM extension" extension located in the Extensions folder of the System Folder. This extension appears near the top of the listing if the extensions folder is viewed by name. You must manually drag this extension out of the Extensions folder since it does not appear in the Extensions Manager control panel under Mac OS 9.