1. Navigate to the /etc/X11/gdm directory.

2. In the gdm.conf file, change #SystemMenu=true to :SystemMenu=false

3. Restart ‘X’ by holding and releasing Ctrl + Alt + Backspace.

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原文链接: http://www.redhatmagazine.com/2008/01/15/tips-and-tricks-how-do-i-disable-the-reboot-and-shutdown-link-on-the-red-hat-enterprise-linux-4-login-screen-from-the-command-line/