[exim] exim / cpanel configuration

Hello to all.. I'm having an issue with people DOSing my server.. here
is the thing.. I can't get an answer to see if I can block ip's like as
the way Denyhosts works with ssh.. I mean.. I have rbls and spammassasin
setup with my exim.. and that all works.. but I want to know.. is there
a way to block people trying to relay through me.. or just allow
authenticated users (valid email addresses on my server) to relay..? I
mean.. this is becoming a pain.. they are sending spam.. and it is
slowing down my smtp which makes my clients mad when they try to send a
200k file and it takes 3 minutes because my server is being nailed to
the wall by these DOS's..

thanks in advance..