Although I'm adding a couple new articles, the link to these duplicate questions are much more complete:
Dupe: How to disable the "Get Windows 10" icon shown in the
notification area (tray)? - read all the answers. – DavidPostill 3
Dupe: What are every Windows 7/8/8.1 updates (KBs) I must skip to avoid Windows 10 upgrading -and nags-? – DavidPostill 1 hour ago
The Microsoft KB article below was referenced from this article:
Latest IE Patch Might Be To Blame for Windows 10 Upgrade Ads - 03/10/2016
By Chris Paoli, the site producer for Redmondmag.com and MCPmag.com.
In a Microsoft blog post released late January, Matt Barlow, general
manager of Xbox Marketing, also included a link for those shops that
want to block the app from appearing on network-connected systems. He
also promised that the GWX will not appear for large enterprises
running Windows Enterprise edition or those that are using onsite
system updating tools.
(Note that the KB link below: https://support.microsoft.com/en-us/kb/3080351 appears to be broken in at least my version of Firefox. I know it works in Edge.)
From MICROSOFT: March 15th, 2016
How to manage Windows 10 notification and upgrade options
Summary: Qualified computers and devices that are deployed in your organization and that are running Windows 7 Pro or Windows 8.1
Pro are eligible for the free Windows 10 upgrade offer and will be
able to upgrade through Windows Update. (This offer is not available
to customers who are using Enterprise or Embedded editions of Windows
7 and Windows 8.1.) This article describes the notification and
upgrade options, and it explains how you can manage these options.