Why?

This also makes no sense to me. Or does your neighbor have access to your WiFi, and is changing the music selection? 

The ‘security’ provided by logging into a Sonos account is only there for adding music sources, it doesn’t protect what is currently playing. Anyone who is on your wifi who has a Sonos controller is allowed to add/change/stop music. 

I suspect what you need to do is change the password in your WiFi, so your neighbor can no longer have access to anything on your network, including the Sonos.

Hi

@Airgetlam is correct. However, assuming your neighbor has access to your network that means one of three things:

Your neighbor is a very good network hacker and not a good neighbor 

You have a very simple (and I mean simple) password and your neighbor is still of questionable character 

You GAVE your neighbor access to your network 

OR you are a Comcast customer and the installer set your MoCa network up wrongly, such that your neighbor is on your local network that way.

Amen to that!!