Never 10 is a free tool which allows Windows 7/ 8 users to prevent their systems automatically updating to Windows 10.
The program is tiny, a sub-100KB executable - just download it and run.
Never 10 is relatively safe to use. Unlike some of the competition, it doesn't disable services, delete folders or anything else: it just gives easy access to an official "don't force-upgrade this system" Registry setting.
Normally this means it's also very simple. Run the program, it'll tell you Windows 10 updating is enabled, so you hit "Disable". If you want to update later, run the program again, it'll tell you updating is disabled, and you click "Enable".
The one small complication is that this setting only works with the latest version of Windows Update. If you don't have this, Never 10 will detect that and offer to download and install it for you. After you've done that, you're able to disable/ enable Windows 10 upgrades as we've described.
Never 10 is a simple and relatively safe Windows 10 update blocker. If you've not tried anything similar then it's a good place to start.