You should remove the upgrade agent if you want to prevent it from downloading the upgrade:
Note that the instructions refer to the "nag" icon but that is just one component of the background system - you should use option 2:
Remove Windows Nag Icon To Upgrade To Windows 10
Microsoft has pushed out a nag tray icon, that never goes away and you cant close it, trying to get people to upgrade to Windows 10.
While it is fine to offer something like that, the point that you cant get rid of it or close it, is what the problem is and shows how desperate Microsoft is to get their piece of the App Store pie that Apple and Google enjoy.
Option 1: One way to get rid of this nag ware is to remove the update that installed it. You must uninstall KB3035583.
"To do this, launch Control Panel, click on Programs > Programs and Features, and in the left side of the screen hit the “View installed updates” section.
Look for KB3035583 in the list, right-click it and hit the remove option. If you don’t want to be bothered again in the future, just hide it and no other notifications will be displayed because the update won’t be installed again on your PC."
Option 2. There have even been others who have found it goes even deeper than that.
"After you uninstall KB3021917, KB3035583 and KB3022345, you also need to disable two tasks in Task Scheduler.
There are two tasks under TaskScheduler > Task Scheduler Library > Microsoft > Windows > Application Experience, "Microsoft Compatibility Appraiser" and "ProgramDataUpdater", that will continue to contact telemetry servers even if telemetry is disabled. These tasks run and phone home even if CEIP is opted-out of. Reproduce (on Win7 Pro)"
Option 3. Simple .bat file script
But if you are a tech and have a lot of computers you now need to remove this from, then going to each machine and removing it, rebooting, then hiding it from the updates is going to be a pain, even if you write a script for it.
To avoid a reboot you can simply stop the exe that is running the tray icon, rename it the folder and be done. I have made a bat file to do this for you, just make sure to run it as administrator of course.