[Exim] System wide filtering

I'm trying to block an auto-reply that is being sent to an alias that is
mapped to several users. I've been looking through the docs and have
discovered that I want a director that's something like this;

central_filter:
driver = forwardfile
file = /etc/exim-filters/${local_part}
no_check_local_user
no_verify
filter
allow_system_actions

What I'm not sure of is how to actually activate it. If I just put it in
the config file how do I get it to work? I suspect that I need to tell it
when to run, but I'm not sure how. I'm still wading through the
documentation, slightly confused by how all these things fit together. I
think I'm getting there though.

Any suggestions? Thanks.

--
Graham Ashton