In Options > Smart Cleaning > Automatically clean on closing without notification
Used to work with a simple test of logging in to Gmail and closing Vivaldi. Then open Vivaldi again and navigate to gmail.com or accounts.google.com and it would force a new userid/password -- the latest version of CC now just opens gmail with the last account used for logging in.
To clean Vivaldi must now press Ctrl-Alt-Delete to wipe all saved items before closing.