The simplest solution right now is to uninstall the upgrade to the Google TV Launcher application under the my apps section in settings for the Nvidia Sheild.
At the same time disable automatic updates in the playstore app, this way the specific TV Launcher app won’t update.
Currently this is the app responsible for showing you ads on your homescreen and playing you full screen ads.
My recommendation is to also downgrade your Nvidia Sheild to 8.2.3, here is a excellent guide below.
Don’t forget about the playstore app updates, these should be set to not autoupdate.
Currently the android TV Launcher app is what shows you the ads on the homescreen. You can easily uninstall this apps update if you upgrade it accidentally.