I have a permanent window box, which I hide with my browser, insisting I upgrade. There is no little ‘x’ to close said box. It’s been there about 6 months now.
I won’t upgrade, because of the subscription that’s reportedly coming. There was an article here at FR that pointed out someone found a subscription-type file in the code; days later almost on cue, MS announced it will be beta charging $7 for ‘enterprise’. So, I’m fine with not upgrading if it means I get to keep MS’s little fingers off my credit card.
Try opening up Task Manager, see if you can see its icon on the Applications tab, if you can then right-click and end task. (You can use the "Switch To" button to confirm it if you're not sure.)
That said, it might not appear in the Applications tab.
You can also try Never10, but I haven't used it and so don't know how well it works.