[exim] Disable exim for local users

Hello all,

On my webservers, users are sending emails with PHP using Exim4. I know
exactly which user is sending as every user using PHP has a system account.
How can I disable an account from sending emails (as local_sender_blacklist
is not working for users sending on 127.0.0.1)?

Is there a best way than writing a script that runs every 5 minutes to
check in exim/mainlog if a user is sending more than say 100 mails per 5
minutes? For example is there a way to put a "quota" with Exim?

Thanks a lot,

Best regards,

Jean