[exim] Replacing header in acl

Hi,

I have the following ACL:
  warn    message       = Subject: *****SPAM***** $h_Subject
          condition     = ${if >{$acl_m0}{0}{1}{0}}
          condition     = ${if <{$message_size}{80k}{1}{0}}
          spam          = nobody
          condition     = ${if >{$spam_score_int}{$acl_m0}{1}{0}}

It works fine, but it adds the new subject instead of overwriting it.
My MUA displays it correctly, but some other clients seem to check the
first subject in the mail when applying which does not contain the
SPAM marker.
Is there a way to make exim replace the subject instead of just adding
it?

--
Adrian