If the following makes your eyes glaze over you should probably click the back button now.

The two registry changes needed are in this text box (Windows 7 and Windows 8.1 only):

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate]
"DisableOSUpgrade"=dword:00000001

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\GWX]
"DisableGWX"=dword:00000001

If you know what you are doing you can go into regedit and make the above changes by hand. Or, you can copy all the text out of the box above, paste it into a text editor, save it onto your desktop as BlockWin10.reg, and double-click it to install.

Here's a reference for this:

http://www.zdnet.com/article/how-to-block-windows-10-upgrades-on-your-business-network-and-at-home-too/

Otherwise, use the utility never10.exe described in the previous post.