Nest audio resumes playing after stop instruction. I have seen other posts from people with the same issue, but have not seen a reply with a fix. Happens with radio from TuneIn and Spotify.
Tap on the left side of Nest Wifi point. 10 total taps will mute all audio except your Google Assistant. Note: This only adjusts the volume of media and your Google Assistant.
I am not sure how your solution relates to my issue of the Google Home resuming music minutes after acknowledging the stop command...
Google Nest Help Center
Chromecast Help Center