[Exim] Spam Reply

Hi i am using Exim 3.35 (debian woody) with amavis-ng and
spamassassin.know what i want to is that every mail wich is tagged as
spam is replied to the sender with a standard text where the sender is
asked to resend his message if it isnt spam with a subject like
foo-nospam.the reply is no problem but how can i set up a filter in exim
that messages with such a subject aren't catched by the spamassassin
director?

greets
             Kevin Moschallski