Postfix block local user from sending
If you are looking to allow certain users access to local mail submission, the parameter you need to use is authorized_submit_users, the only issue is that it is a whitelist, hopefully you don't have lots of local users who need to send email. If you have postfix 2.4+ you can use a format like this: authorized_submit_users = !joe, static:all
which is joe no, everyone else yes.