The problem is not Nabble. When you subscribe to a mailing list (even when it has a forum front-end) you start receiving mails from there. If you want to leave, you should contact the mail list owner and not the mail list provider. It's like if you join a Yahoo or Google Groups mailing list and then write to Yahoo or Google complaining about receiving e-mails.
Anyway, you might want to try here:
http://xircles.codehaus.org/manage_email
Nabble staff. We never ask for passwords.