How do you prevent wuauserv (Windows Update Service) from restarting?

Regarding:

net stop "windows update"

this works for a while but then the service starts again. What is restarting this service and how do I prevent it from doing so?

Stop it and disable it.

sc stop wuauserv
sc config wuauserv start= disabled

You will need to open command prompt as administrator or you will get "access denied." The space after the start= is mandatory, sc will complain if the space is omitted. It may be worth noting that any services, or programs, which depend upon a disabled service will themselves fail to start, or experience anomalous behavior.

Check the System event log for event ID 7036 from Service Control Manager for wuauserv entering running state. It's likely another service has wuauserv marked as a dependency and is starting it as needed.

Windows 8 have a Scheduled Task:

Task Scheduler Library
:---- Microsoft
      :---- Windows
            :---- Windows Update

There is a task: "Scheduled Start". Just right-click on it and choose Disable.