[Exim] ACL to Reject based on Subject

Hello all,

I have been reading the online manual and searching on Google but I can't
find anything that will do what I am looking to do.

Using an acl_smtp_data I would like to scan the subject line of the
incoming message. If it matches against a list of keywords (like viagra
or mortgage) Exim would refuse the message.

Is this possible? If so, can anyone point me in the right direction?

Thanks in advance,
Lane