You should have received a confirmation email from MS stating,
You’ve just reserved your free upgrade using the “Get Windows 10” app. You can also use the app to confirm the reservation, check to see if your device is compatible with Windows 10, and un-reserve if you wish to cancel your reservation prior to your Windows 10 upgrade.
As I recall the instructions for unreserving were in the instructions for how to reserve. I realize that may not be much help, but now you know there is a way to do it. Try googling “unreserve windows 10.”
Came back with the same answer I got from MS. At least they’ll ask first if you want to install.