Those ads are probably displayed by google's launcher, you can easily disable it from adb and use any other launcher you like. I personally use FLauncher, I love how simple it is, I don't need anything else
Also for anyone readying this in the future, you can find the android tv launcher app under apps and uninstall it's updates, then in the play store set apps to not autoupdate. This prevents the launcher from updating to its newest version which has the ads play through it.
Found this out while downgrading my sheilds to 8.2.3 and when the launcher app updated I still saw adds.
Here is a well written guide how to downgrade to 8.2.3 that I followed if interested. It's not required but I find it's alot more stable.
I've heard you can flash lineage on there... Media PC in the main room but I've always loved my shield for a spare room control. Would rather risk bricking it by flashing it than dealing with ads.
They can't even do the "poor Google we have a service to run" thing here, this is just greed. I bought the hardware, I don't deserve ads on it too.