Whatever app you used to start streaming, you would use the same app to press the Stop button. Or you could issue a voice command to either your Google smart speaker (if you have one) or to your Android phone, asking your Chromecast Audio (or however you named it) to stop.
A couple things:
1) I'm not sure if this is Operating System / Ecosystem dependent, and I'm also not sure if it does/doesn't depend on whether you have the Google Home app installed on the device or not, but what I can tell you is that I'm on Android (for phone and tablet), and I have the Home app installed on both devices... and if someone, ANYONE, is casting something on the network using an App other than Roon, it shows up in my dropdown trey that there's a stream going on. I don't think this is JUST limited to the Google Home's but also includes the CCA's? I'm not sure but I think so. Mostly I just see notifications that one of the kids started casting to one of the Home's.... they don't know how to work any of the CCA's (yet).
2) As other posters suggested, I think it would depend on what kind of "play mode" you're using. As long as you don't set the album, queue, or playlist to "repeat continuously," and you aren't using a "radio" feature that just starts queuing up endless songs (whether it be something like Roon Radio or Google Play Music's "stations"), it should just play every song currently in your queue (whether that be an album, a playlist, or a manually generated queue) and then stop...