If you removed a speaker and it still appears in the app, just wait a couple days ant it will disappear.

From the app? I think you would unplug it, then reboot / power cycle your other Sonos speakers and your router, both of which would ‘remember’ it for a while until flushing it out. Or just wait, too. 

From your account on Sonos.com, you’d have someone new register it to their Sonos account (one of the reasons you should factory reset before passing it on, forcing the new user to register it to their own account).

Unfortunately a simple “remove” button is far beyond the capability of Sonos’ software team, so you just have to wait a while.

Though for some such a button my be nice, we do not know if adding such a button would be simple.

As a software engineer with 25 years experience, yes I do know. But thanks anyway