GWX Control Panel is a free tool which can stop Windows 7/ 8 nagging you about upgrading Windows 10 (or maybe doing it automatically).
Launch the program and it displays various details about your system status. You'll see whether the "Get Windows 10" icon app is running and/ or enabled, if automatic Windows 10 upgrades are allowed, if you currently have a hidden Windows 10 Download folder with installer files, and more.
A group of buttons provide easy access to various tweaks. You can enable or disable the "Get Windows 10" app, prevent all automatic Windows 10 upgrades (this doesn't stop other Windows updates), delete any hidden Windows 10 download folder, and more.
There have been reports that Windows can change some of these update settings back, without asking first, but GWX Control Panel tries to counter this with an optional "Monitor Mode". Turn this on and it tracks your Windows 10 settings, raising an alert if there are any changes.
GWX Control Panel is a well-designed tool which gives you a lot of control over the Windows upgrade process, but does this intelligently (not just globally disabling Windows Update), and provides excellent documentation to help you understand what's going on.