[Exim] removing headers only for incoming SMTP

I have gone through the FAQ and the manual, and can't, for the life of me,
figure out the right way to either run a router only upon receipt of
incoming SMTP messages, or to explicity remove headers.

Spammers have finally caught on to the fact that many installations rely on
various headers to keep track of if a message has been scanned as spam.
Because of this, I wish to specify a list of headers to remove when any
message is injected via incoming SMTP.

Any suggestions are welcome -- perhaps this should also go into the spec
and/or FAQ.

Thanks,
Roy