Now it's got to the point where they're beginning to implement forced upgrades. Today when I declined the nag dialog for the umpteenth time, I got this message:
YOUR WINDOWS 10 UPGRADE HAS BEEN SCHEDULED FOR WEDNESDAY 18TH MAY AT 04:00
So, obviously, they plan to go behind my back and install the wretched OS while I'm sleeping. In a fury, I screamed about this to an IRC channel inhabited by jokers and techies, and one of them came to my rescue. A company called Gibson Research Corporation is offering a free registry editor called Never10, which bars 10 from installing itself. Note that I call it a registry editor rather than an app, because that's what it is. If you use it, nothing is installed on your system.
As a bonus, Never10 scans your system and detects Win10 files that have been "pre-loaded" onto your system already. When I ran it, it found six point five gigabytes of these bloody things, which I promptly deleted.
Here's the link -- it worked for me and you may wish to try it if you don't want your screen to resemble something more appropriate for nursery school, and your net activity easily monitored by faceless droids in Redmond WA. With grateful thanks to my knowledgeable IRC buddy.
Update 19 May
It wasn't enough. I was hit by Win10 overnight. I'm furious.