GWX Control Panel (previously named GWX Stopper) is a free program that you can use to configure and exit the “Get Windows 10” system tray application which continually pops up on PCs that are still running Windows 7 and Windows 8. It can also prevent unintentional Windows 10 upgrades from occurring via Windows Update. GWX Control Panel really works, is safe and easy to use, and gives you the option to re-enable the icon and upgrade notifications if you’re ever ready to move forward with Windows 10.
UPDATE (November 1, 2015): Due to popular demand, version 1.5 of the program now includes the ability to delete the hidden Windows 10 installation folders (Windows.~BT and Windows.~WS). There are a few other features intended to improve the user experience and assist tech support staff who use GWX Control Panel to remotely help customers. There was a LOT of under-the-hood work done in this release- please let me know in the comments or on the Ultimate Outsider Facebook page if you see unexpected behavior.
Download | Direct Link