Change your Sonos password and delete the Sonos app from their phones.

Make a second network with just the Sonos on it so they came find it, then delete the app from their phones after lol

Get a travel router, like you’d use in a hotel and move your Sonos to it, also connect your controller to it.

That keeps the family off your Sonos and if you need access to something on the house network you can just switch your WiFi connection to access it.

It then depends how determined they are … there is nothing to stop them downloading the app again and connecting to the system.  Not knowing the password would prevent them from changing settings (which might be worthwhile in itself), but would not stop them using the system.