A simple, but easiy defeat-able scheme, would be to use a switched power outlet. At the other extreme would be to physically modify the unit and install a key switch.

If you are capable of DIY network control programming, you could brew up a network monitor that would watch the ARC and stop its play during unauthorized hours.

Why do you need to disable ARC? What problem are you solving?

I want to stop others in the household using it whilst I’m away from home 

can I do it via the app & wifi

All methods could be defeated, depending on how determined your housemates are, and how willing to openly ignore any requests by you that they should not use your Arc.

You could get a travel router and set up a dedicated LAN for your Sonos and your other networked devices.  Only you need know the network password.  This could be overcome by factory resetting the Arc abd setting it up again.  But if they are that disrespectful of your personal property you should find some better housemates.

Or you could take the power cable with you when you go away.

Disabling WIFi for ARC will prevent it from playing music, but it will not disable the HDMI connection and its Autoplay.

Blocking a determined teen, such as myself (I’m a perpetual teen), would be a nearly futile exercise.

Is there any context to this that might help people give more specific advice? 

Disconnect it and take it with you. 
 

This method is pretty much 99.9% reliable in preventing use whilst you’re not there. 
 

Whatever method you use though, it does seem rather petty. 

I’d love to see the 0.01% scenario explained where they can still use it even though it’s not there! 

That’s statistics for you And where’s the other 0.09% gone? 

I wasn’t sure how to calculate factors like theft of the speaker by other members of the household. 

Brilliant. (And sorry, I read it as 99.99%, hence my mathematical faux pas.)

The housemates could also buy a different Arc and replace yours with the new one. Whatever efforts you do to disable the Arc, would be on the replacement Arc, not technically your Arc.  Thus, the housemates could just plug your Arc back in when you’re not there and use your Arc.

All my other ideas involve kidnapping or time travel.

Now you’re just being silly

I do need to correct my figures though. In the UK in 2021 ( the most recent year for which the data is available) there were 17,689 reported thefts of soundbars. Of these, 11 were attributed to other members of the household - mainly students and people in shared accommodation. Allowing for the number of soundbar manufacturers and the market share Sonos enjoyed during that year, I’ve revised my calculation to 99.93%. 
 

Please note that, like many statistics, my numbers above are purely fictitious.