The sponsored rows, the big promo banner and the "Shop" picks all come from Google TV's home screen app. You can hide most of them in a minute from the TV itself, or remove them completely with a computer.
The home screen now shows your apps and little else. What you lose: the For You tab, personal recommendations and the Google Assistant's suggestions. Tracking services keep running in the background, and some TV makers' own promo rows stay.
A different launcher has no ads at all. On Google TV, though, the Home button keeps returning to Google's home screen while it is enabled: changing the default home app does not stick on several devices. The reliable way is to install the new launcher first and then disable the stock one, com.google.android.apps.tv.launcherx, with ADB.
com.google.android.tungsten.setupwraith) takes over Home once the stock launcher is off, and disabling it can break casting from phones and delete your saved Wi-Fi. Check your device page before you do this.Open source launchers built for this, such as CleanYourTV and Free My TV, come with install scripts and a one-step restore.
Keep Google's home screen but switch off the packages that feed it promos and collect viewing data. This removes TV-maker promo rows, pushed messages and auto-installed apps, and it is fully reversible. Use the tested list for your device, or build your own with the command generator.