Perhaps the manage your account FAQ would help?
Generally, when moving a Sonos device from one account to another, a simple factory reset should suffice, as it forces the new user to add it to their own account, and removes it from your account at www.sonos.com. It may take a day or two to disappear from your controller, though, since those two processes are not directly linked. I think there is a ‘forget this device’ that shows up when a device is ‘missing’ in your controller, though, and a power cycle of your router may not be remiss.