There was an app update today that included the following fix, if this might help:

Added the option to forget a system in order to connect or create a new system

Might be drastic to forget the system and set it up again but it may be the immediate solution.

Where are you seeing them? 

If it’s on the list of your devices at www.sonos.com/account, they won’t disappear until the ‘new’ owner factory resets them, and adds them to their own account. If it’s merely in the Sonos controller, @Rhonny ’s suggestion above is likely to be a good answer, although those devices should drop off at the time your router renews its leases, and the data is no longer cached on each speaker. 

thanks for that information.  I no longer communicate at all with the “new owner” to tell him to do a factory reset and create his own account.  With that, I suppose I just have to live with it.  Sonos should give control for us to remove devices, I mean people do sell these products to others, especially if they upgrade devices.   I doubt this is a rare situation. 

One other question: if his devices still show on my online account (not on my app) would he have access to my music accounts I have associated with Sonos?  

On your music services, sign out of all devices. Then change your music services’ passwords, and I think all access to your services on their speakers should cease. (Someone jump in if I am incorrect here.)

The players will stay with your account until someone else adds them to another system. It seems silly, but if the players are trashed or ignored forever, you will remain the owner.

All former shared accounts should be brought under your control by immediately changing the password. You may need to close some accounts in order to terminate any reference to your ‘ex'

With regard to units nolonger present in your system, they will eventually stop being listed in your controller or you'll be able to indicate that they be ignored.

If a player that has been removed was half of a stereo pair, it will be more difficult to ignore. I recommend that the remaining player, and only that player, be factory reset, then added to your system as a new player. Take care to select the proper option, don't accidentally setup a new systen.

Thanks everyone for the advice.