It should just disappear after a while. 

I would hope but I need a confirmation of this. I see no reason why sonos doesn’t make it easy to remove a device.

I have an old Play 5 that is still there 5 years later.  If the device is registered by another user it removes it from your account.  If not it will stay.

Thanks.  Crazy that you cannot unregister it.

If it’s showing up in your router / on the app, and you’re really worried about it, try a reboot of both your Sonos devices and the router, to ‘flush’ the temporarily held assignment from your system. As @nik9669a suggests, they’ll drop off there as your system re-leases the IP address, in time. 

If it’s on the ‘I’ve purchased these devices’ on the www.sonos.com page, it will stay there indefinitely, until such time as it gets registered to another Sonos user, as @bockersjv suggests. I’ve got several items on that list, mostly old BRIDGEs that I didn’t give away, but junked. 

Thanks

“Selling” them to yourself, using a throw-away e-mail might be an option.

wouldn't I then need a bogus Sonos account just for trash devices?

Yep, but they are trash so why would you care?

Set it up, do the sale, dump the e-mail account.

Sure there should be an easier way.

https://www.startpage.com/sp/search?query=disposable+e-mail+address&cat=web&pl=opensearch&language=english

I think TS is not talking about his account, but about the app?

@AHN 

Like @106rallye said that are different things to see a device in the app or in your account system data via web page. 
I can tell you that a not connected device definitely will disappear from the app after 72 hours. 
In your account as someone told before it will stay as long as it’s registered with another Sonos account. Sonos also can’t change that manually if you are going to ask them.