It's a question without a good answer. There doesn't appear to be a "permanently prevent Windows 10 upgrade" switch anywhere.
If someone made a tool that lets me use Windows 10 with security updates but without spying or cloud or unwanted upgrades, I would pay for that. I don't see any technical reason why a 3rd party can't provide that. When Windows 8 messed up the start menu, tools like Classic Shell stepped in to fill the gaps, with huge popularity, and I think those download statistics were actually persuasive to the "data driven" business strategizers at MS.