[Exim] defer delivery to one host

I'm using exim 4.04 on a RH7.1 box. It is just a mail gateway with
no local deliveries.
We need to convert mail from one system to another on our main mail
server (Pegasus Mail on Novell to a GroupWise server that has been
acting as a POP server).
We need to keep SMTP running on the GW server but need to stop
outside mail from being delivered from the exim box during the
conversion
process.
However we would like to keep mail being delivered to other hosts
and to allow our dial-up users to send out mail.
I see from the FAQ Q5020 how to stop all deliveries, but i just would
like to stop delivery to the one host.
Thanks,
gerry