In order to properly disable the Welcome to Office prompt, there are several settings that need to be configured in GPO before this takes effect. Most articles online tell you about just one setting that needs to be changed, but this is unfortunately not enough. The Welcome to Office prompt does more than just configuring updates, it also sets up the "Customer Experience Improvement Program" (CEIP) and Office Diagnostics. Without configuring these settings in your group policy, the prompt just appears anyway.

To set this up, you'll need the Administrative Templates for your Office version:

Office 2010

Office 2013

Office 2016

How to use these is out of the scope of this article  ...