To stop a Spotify Connect connection to your Sonos, play any other streaming company’s stream. However, once he gets back into your Wi-Fi range, he’ll be able to re-initiate the Spotify Connect system. 

In the case of the Spotify App playing through a SONOS system, Spotify is working through its cloud to control the local SONOS system. When the phone/pad is taken to a new location the cloud/SONOS system connection is retained. While I view this as a fault, someone thinks this is a feature. The Spotify cloud is very aware that the phone/pad has moved out of direct listening range. Perhaps someone thought it would be a nice feature to be able to change music for the cat or a burglar while at work. A more legitimate reason would be the ability to start play from the car before entering the house.

I just choose to play something else to the speaker, thus replacing the Spotify API connection as mentioned by Bruce  - it’s also possible to go to the ‘now playing’ screen in the App and ‘end the session’. See screenshot.

I’m not sure why such sessions do not auto-expire after a set period of inactivity, but maybe there is some technical reason why that doesn’t happen.