How to delete previous owner's station listed in the SmartControl dropdown?

Here you go:

When selling a FLEX-6000, it is desired to completely remove any user-configured data or settings from the radio before transferring the radio to the new owner. The following procedure will outline how to remove all user-configurable data from the radio.

1.) Unregister the FLEX-6000 from your SmartLink Account

• Start SmartSDR for Windows. If you are not logged into your SmartLink account, login now.
• Highlight your radio and click on SmartLink Setup
• If you see a green check mark and it indicates your radio is "Configured for SmartLink", click on the blue UNREGISTER button.
• You will get a message that the software is trying to connect to the SmartLink server. When a connection is made, you will get the message "Hardware Key the radio now".  Press a hardware PTT (foot pedal, mic button, CW paddles). 
• The message "Your radio has been unregistered" will be displayed when you have successfully unregistered your radio from your SmartLink account.

There is no need to delete your SmartLink account even if you do not plan to use it ever again.

2.) Reset The FLEX-6000 Callsign and Nickname

• Connect to the radio using SmartSDR for Windows. From the top menu bar select the Settings -> Radio Setup -> Radio tab
• In the CALLSIGN field beside the Callsign button, backspace to delete any information in the box and then enter NOCALL. Press the <ENTER> key when done.
• In the NICKNAME field beside the Nickname button, backspace to delete any information in the box and then enter FLEX-6000. Press the <ENTER> key when done.
• Close the Radio Setup window.
• Shut down SmartSDR for Windows.