[exim] Exiscan-ACL, SpamAssassin, Subject header

I have recently upgraded Exim to version 4.43 and with that installed the Exiscan-ACL patch so that I can scan messages for viruses and spam in the data ACL so that the bad messages can be rejected rather than accepted and then trashed. Everything seems to be working fine except that where I thought I was rewriting the Subject header, it is only adding another Subject header. I followed the examples provided in the Exiscan-ACL documentation by putting the following in the data ACL:

    warn  message = Subject: *****SPAM***** $h_Subject
            spam = nobody

Is there a way to re-write the Subject header in the ACL or would I need to do it in a router or transport? Would I be better off configuring SpamAssassin to rewrite the Subject header or would that interfere with the ACL?

BTW, I'm using SpamAssassin 3.0.1. It works great.