This is not an option in the Device.Backup.Running Config activity as the passwords form part of the backup.
It would be possible to run an external script from the Device.CLI.Send commands activity after the backup which removed the passwords but it would be down to you to craft such a script. You can see some examples of external scripts in the Thwack content exchange. Please feel free to post such a script in the content exchange should you create one.
From a Networking perspective you would better off pushing out "service password-encryption" statement through Kiwi to encrypt your passwords.