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.

florisse.nl/shield-downgrade/