All you need to disable is: Windows Update (wuauserv), Update Orchestrator (UsoSvc), and Windows Update Medic Service (WAASMedic), and it will stay disabled. But you have to do it with the registry editor.
Find those services in: Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services and change the ‘Start’ value to ‘4’, which is disabled
It will whinge about permissions so you’ll have to right click on the folder for each service and grant yourself permissions to edit it.