... it kept highjacking my computer to install things without my permission and at inconvenient times.
You Won't be able to stop updates entirely, but with the Anniversary update build you'll be able to set a time window for updates, & 10 won't try to update outside of that window.
Windows 10 updates itself through Windows Update, and it also updates the default apps through the store. You can uninstall some apps using 10's Settings, and most others using CCleaner -- the fewer apps you have from the store, the less time it will spend updating them. Windows Central has a long list of tutorials on 10 [ windowscentral[.]com/windows-10-help?utm_medium=navbar&utm_campaign=navigation&utm_source=wc ], like this one that shows how to uninstall apps using Settings [ windowscentral[.]com/remove-suggested-apps-windows-10 ]
I am being nagged to install a Windows 10 upgrade... I see something about the free Windows 10 upgrade expiring on July 29, but I already have it on my computer.
I think I'm about to convince some more people that they're Never, EVER, going to upgrade to 10...
I *think* you're **probably** running the initial Windows 10 release from the end of July, 2015. There was a new build last November [v. 1511], & there's another coming August 2nd. AFAIK new builds are the only updates you can refuse indefinitely. Also, since the new build is coming 8/2/16, if MS had every copy of 10 expire 2 days before that, the chaos would be of epic proportions.
Now backing up a second, what's a build? A new build of Windows 10 is a completely new version. You can't just update Windows 10 to a new build -- though it's delivered via Windows Update -- but have to completely reinstall Windows. The way it usually works is Windows Update will download a new build, run it to get things ready, then tell you you have to restart -- if you restart Windows 10 normally [e.g. right click the Start Menu button] it won't upgrade, so you have to click the restart button on the Windows Update dialog window.
Then when Windows 10 restarts it starts the installation/upgrade process, almost as if you were installing Windows from a USB stick -- you'll see a couple of progress screens but not much else. How long it takes depends on how much software you have installed & how fast your device is -- with not much software installed my very low powered tablet takes all afternoon, while this more powerful PC can do it in less than an hour.
It's a good idea to have a full disk image backup beforehand, & it doesn't hurt to have another device handy that you can use to go online for help, because there is a [*usually* slight] chance that your device will not be 100% compatible with the new version of 10. That's what happened with my tablet & the 1511 build. MS claims that this is now fixed, but what happened was 10 would roll back to the old version or build, download the new build again, & start the upgrade again, which would also fail, & so on & so on.
What happens next?... Well eventually you're back in Windows 10, & if you set Windows Explorer to view hidden & system files, you'll see a Windows.old folder with your old version of Windows, along with some [or all] of your previously installed software. You 1st need to make sure that all of your software made it to the new version & works -- otherwise you might have to move it out of that Windows.old folder back to where it belongs, or reinstall it.
You also have to go to Control Panel -> Device Mgr. to see if there are any problems, plus check to make sure everything is working, like cameras & audio. If not, go to the problem device(s) in Device Mgr., right click it, & have it check for online updates. If that doesn't fix it, you have to find the old driver in Windows.old\ Windows\ System32\ Driverstore\ FileRepository\ -- once you find it you have to update the driver again, only this time selecting let me choose the driver, clicking the button for have disk, & browsing to the old driver you just found.
Now that you're sure everything is working, hardware & software, you can get all your drive space back, because the upgrade likely left somewhere between 12 & 30GB of junk behind. Note: if you don't have much free space on the hard drive, if you plug in a USB stick or microSD card before you do the upgrade, 10's setup is supposed to use that. And getting rid of the junk is easy, if it works -- right click the Windows partition or drive letter in Windows Explorer & click Disk Cleanup.
The progress bar will run for a while, & then a dialog box will pop up -- click the button to include system files, & the progress bar will run again for a while. Once the dialog box pops back up, select every check box & click OK -- you'll see another progress bar, get a message prompt asking you are you sure, & then it'll run for a while & just disappear when it's done. Now's the time to cross your fingers, say a prayer or 3, and see if Windows.old is gone. If it is, great -- just clean up the stuff Disk Cleanup missed in the 2 temp folders & the 2 new, useless icons on the desktop.
If Windows.old is still there the easiest route would be to ignore it & see if it caused problems in the future. *In my experience*, getting rid of the files in Windows.old has meant changing ownership twice, renaming the files & some of the folders to make it easier, then booting to a command prompt [I use my Macrium Reflect Free USB stick], and deleting each file, then the folders they're in, individually. NOT FUN.