Use WhatsApp's own log-out, open your WhatsApp in Spanbox and use the same 'Log out' option you'd use on WhatsApp Web (in WhatsApp's menu). That ends the session but leaves WhatsApp sitting in your Spanbox sidebar, so you can scan the QR and sign back in whenever. Your chats live on WhatsApp itself, so nothing is lost. If you'd rather wipe the session entirely, remove the app and re-add it.