What ninja upgrade? You always had to opt-in. Yes, they were really pushing the offer annoyingly hard, but I had no problems whatsoever to keep one of my machines on Windows 7.
Anyway, you can stop doing so now, the time for a free upgrade is over.
Not a solution for those of us who run Windows boxes for various reasons...
And to clarify, I plan on occasionally letting updates through (I'm already on Windows 10) but this is a great way to prevent data collection / backdoor activation, which I hadn't considered. Seems like the simplest way to add a lot of privacy to Windows.
And considering the Windows 10 upgrade was being pushed through Windows Update I'm not sure how you'd want to prevent that specific update by blocking an IP and not interfere with Windows Update as a whole.