You can right click the Start button -> Settings to begin customizing the menu as you want.
If you use the Classic styles, you can edit the menu items from the Customize Start Menu tab to keep only the shutdown commands you want. You can change their order and remove anything you don't want.
For Windows 7 style, there is a "Shutdown menu items" setting but it offers less customization compared to Classic styles.
Once you customize the menu to your liking, you can set the appropriate Group Policy setting to lock it down or edit the Registry. Look at the Administrative Settings section in the Help file for how to lock it down using the Registry.
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how to customize menu items & enforce them via Group Policy