I believe the Spotify service will find all the devices (of varying brands) on the local network subnet. To stop that happening, perhaps simply add your own router and create your own local network subnet. That’s seems to me to be the easiest way to resolve your issue. 

It would be a relatively simple network discovery process, I’m not aware that either Sonos or Spotify would enable you to ‘edit’ the results. However, since Spotify owns that software, you may get a better answer if you were to ask them, since Sonos in this case is only responding to a request. 

Great, thanks all for the answers!