Sorry, my bad. Somehow I was thinking of you wanted to keep the switch user option, but I get what you meant now.
In this case, go to registry editor instead. Then find this:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\PolicyManager\default\Start\HideShutDown
On the right pane, change the "value" name to a 1 value.
This will affect the admin user as well. But you can always change it back to 0 if you ever need to turn off the computer.