The easiest way to stop Windows 10 upgrade is to go into the Windows Update application, then select Change Settings, then set it as "Never check for updates" (this is for Windows 7, the wording may be different in Windows 8).
Then it will never upgrade to Windows 10.
Every so often (weekly?) it will then be worth going in to Windows Update and press "Check for Updates" to see what Windows updates are available.
You can choose to deselect the one to update to Windows 10 but install the rest.