Stop Xbox One controller from controlling Windows 10

Turns out it was the new Steam "Xbox Configuration Support" feature. I disabled it and now the controller doesn't do anything in Windows and all my games work again.

Before (with Issue):

After (Without Issue):

If you want to keep Steam's "XBox configuration support" enabled (which is useful for things like calibrating dead zones + some games require it) and still disable the controllers from controlling your mouse and keyboard:

1. From the steam main application (not Big Picture) click "Steam" in the upper left corner
2. Click "settings"
3. Click the "controller" tab
4. Click Desktop Configuration
5. Select each of the fields that say for example "Mouse button 1" or "Left ctrl" and use your xbox controller to press the button for "remove" (I had some issues using the mouse in this menu). Some fields have sub-fields and you need to remove those too.
6. Press the back button (default: (B) ) on your controller until you exit the special menu.
7. You're all done :)

If none of the other answers work for you, I found a definitive solution to this maddeningly dumb issue. Note: I'm translating this from my native language, there might be discrepancies in the menus' names.

• Open the OG control panel (Windows + R, control, Enter)
• Search mouse, click the big green menu entry (likely to be the first one)
• Go to the Hardware tab
• In the list view named Devices, there's a bunch of HID-compliant mouse entries. Find the ones whose Location property, in the area underneath, would indicate that it's actually your XBox controller. It definitely helped that my XBox controller was plugged in to my keyboard, so the Location property's value would conveniently read SteelSeries Apex 7. I suggest you do something similar in order to have an easier time discriminating which is which.
• Once you find one of those, select it. Then:
  • Click the Properties button at the bottom of the window. In the window that opens, click the Change settings button at the bottom to reopen the same window with elevated privileges.
  • Go to the Driver tab, and click on the Disable device button near the bottom of the window.
  • Now you can go back to the list of HID-compliant mouses and do the same thing for every other such entry that you find. I had two of them in total.
• Then, you can go back to the control panel, search keyboard, open up the big green menu entry named exactly that, and do the same thing again with the list of PIH-compliant keyboards in the Hardware tab. I also had two of them here. This time however, I wasn't able to disable the device drivers (button greyed out), so I straight up uninstalled them (button just below). My keyboard did act up for a couple seconds at that point, so I fiddled around, unplugged my XBox controller, and then it started working again. Not sure whether the unplugging had anything to do with it, but worth mentioning just in case.

This fixed my issues entirely, and it looks like a pretty durable solution.

Mandatory rant:
TBH it's borderline unacceptable from Microsoft that there isn't an accessible toggle for this one particular thing on Windows. Due to a crappy USB cable, my controller would frequently disconnect while playing, causing the driver or whatever to choke on the input, and next thing I know I can't browse ANY of Windows' Metro apps, including the start menu, the Settings app, and just about all sorts of basic functionality: they just start behaving as if I had the Tab key or an arrow key pressed down.
I had tried everything prior to this, only a reboot would fix it. Pretty stupid behaviour IMHO.