DisableOSUpgrade is insufficient
I have tested this extensively as I'm kinda on a personal war against Microsoft right now after one too many upgrade prompts experienced by my friends. While the "DisableOSUpgrade" registry value does indeed help to block the upgrade process it does not actually block the constant "OMG PLZ UPG NAO!!!" nagging caused by the KB3035583 update. And as I have posted recently, you may not wish to uninstall KB3035583 as its removal is known to brick Windows Update notifications.
To shut that one up you need to use:
This isn't documented but it is known and it works. There is also another value named "ReservationsAllowed" which is mentioned in the same article which you may also want to look into using.