I disabled sending mails in the disk quota module a week ago, since I scheduled warnquota as a cron job, and mails are created by warnquota now. But webmin still sends mail twice a day. Plus the user that was over quota has reduced his data and is under quota now - but still two mails per day for him.
BTW, the mails are undeliverable, since they are not addressed to the local user, but to localuser@my.domain, so that the ldap query of postfix does not kick in. (it does for the warnquota mails). Unfortunately, I can't leave away the domain part for the addresses in webmin.
Are you sure quota notifications are disabled for users and groups on all filesystems? They can be enabled separately for each filesystem and type ..
You can forcibly manually disable quota monitoring by editing /etc/webmin/quota/config and removing all lines starting with email_