Maybe put the relevant Ports on a ‘timer’ power-switch and power them off/on completely to your chosen schedule. That sounds like a better/easier solution to me.

Sonos certainly doesn’t offer a feature in their software to do this. I can see the value of the request, but it really seems like better training of the users might be a better solution. 

Have you tired to teach every guest how to use Sonos to play some song that was cool that they wanted to hear at party at night?  Great UX is not having to make the user think and understand how the software or system is implemented or deployed.  Someone who may not be 100% on point can safely play music. That’s the job to be done. Not training on implementation details. 

For Sonos this is also about adding complexity to the app and the system. Providing this feature will create a need to ad buttons in the app, which will result in people misunderstanding them, calls to the helpline and posts here (see, for example, the posts about people not being able to form named groups when they only have two rooms in the Sonos app).

That's not to say Sonos won't do this, but that they have a broader scope when adding features.

There are some automation tools that work on Sonos for iOS that could be associated with a timer that stops playback at a certain time.

There’s no way to block someone starting new music on your external speakers at an ungodly hour though, except through control of the power to the Amp.

I think the new Sonos for business is supposed to handle these sort of situations.

https://www.sonos.com/en-us/sonos-pro

I realize you aren’t running a business, may not have need for many of the features available, and probably don’t want to pay a subscription fee, but this is how Sonos has structured these features.