I know of three Windows update files that are related to the nagging about "upgrading" to Win 10. They are KB3021917, KB3035583 and KB3123862. I believe that the last one is related to the "behind the scenes" automatic downloading of Win 10. There may be others, but by uninstalling and hiding these three I do not get any messages on my Win 7 machine to install Win 10.
I made a post back in December describing a method to uninstall and hide these updates so they won't install again. When I checked Windows Update before writing this post, I found that KB3035583 had again appeared as a Recommended update. I hid it again so it wouldn't install when I do other updates.
As others have said, the Windows Update settings need to be set to "Check for updates but let me decide whether to download and install them." Don't choose "Install updates automatically."
My December post.As I said in that post, Microsoft really, really wants everyone on Win 10. Just my opinion, but I think Microsoft's new business model is to use Win 10 to collect user data and sell it to advertisers as a new source of income. Why else would they want everyone to have an "Advertising ID?" Google and others have been doing this for years and IMO Microsoft wants a piece of the action too.
BTW, I have a Win 7 desktop and a new laptop that came with Win 10. I turned off Cortana and turned off the other privacy issues in settings and find Win 10 to be OK. I still prefer Win 7 as it just looks better on the screen.