Frankly, this entire episode has been indefensible, with Microsoft introducing a non-stoppable, non-hideable advertisement on several hundred million PCs from around the world. And then upgrading that advertisement to thwart those who do seek to remove or hide it. It has changed the language of the ad, made no clear cancel choice available, and jammed it into the “recommended” updates that auto-install via Windows Update. …
This time, it actually changed the behaviour of the window that appears so that if you click the “Close” window box, you are actually agreeing to the upgrade.
I stopped being a Windows user years ago due to user-hostile behaviour like this.