Create a .bat file on your desktop and name it something like "Rift S stop", or something. Then type the following
net stop OVRService
Save it, then just right-click the file and Run as Admin. The services are now stopped and the Rift S won't come on.
To start the services just do the same, name it something like "Rift S start", and type
net start OVRService
Save it, then just right-click the file and Run as Admin.
As soon as Windows boots to the desktop, I always stop the services until I want to use the Rift.