To Disable Lock Screen and Log Out

GNOME 2.6 Desktop System Administration Guide
Disabling GNOME Desktop Features

To disable the lock screen and log out functions, set the /apps/panel/global/disable_lock_screen key and the /apps/panel/global/disable_log_out key to true.

When you disable the lock screen and log out functions, the following items are removed from the panels:

Lock Screen and Log Out user menu items from the Main Menu.
Lock and Log Out menu items from the Add to Panel ▸ Actions menu. To open this menu, right-click on a vacant space on a panel, then choose Add to Panel ▸ Actions.
Lock Screen and Log Out user menu items from the Actions menu in the Menu Bar applet.

Also, any Lock Screen buttons and Log Out buttons on panels are disabled.