[Exim] enable host_reject

Hi All,
I'm a new guy to play with exim. I installed an exim of version 3.33
running on a box which has RedHat 7.0 OS. It's working fine. Now I'm going
to stop some unwanted e-mail from internet. I know the "host_reject" option
may help. But I can not see it on my run-time config file. So what should I
do to enable it ?
Thanks for any help.