[exim] ACL condition to block a user from sending out mail

Hi,

When ever mails are sent via script, the mail log will be something like:

-------------------
2013-09-10 07:46:56 1VJIf2-001qEs-6n <= user1@hostname U=user1 P=local
S=3773 T="Vakante Stellenangebote" for email@???
-------------------

Is there any way I can suspend the email functionality for user "user1".
For example, if a user is trying to send out mail as user1, the mails
should be discarded.

After checking exim doc, could find there is a parameter $authenticated_id,
but this isnt working under acl acl_not_smtp. And mails sent via scripts
uses that acl.

Is there any way I can achieve this.

Thanks in advance.

--
Regards,
Soumya