[exim] acl to deny senders

I have a number of local users that I wish to block from sending email.

I have the acl below in place and when I test with exim -bs I see the deny
message but when the local user sends an email it is delivered.
The aliases.deny file has the users email address
What have I overlooked?

acl_smtp_data = acl_check_sender

begin acl
 acl_check_sender:
      deny message = "Please use private email"
     senders = /var/exim/data/aliases.deny

Thanks
Judy Angel
University of Hertfordshire