To disable the lock screen and log out functions, set the /desktop/gnome/lockdown/disable_lockscreen_and_logout 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.