[Exim] How to restrict certain users to local delivery

Hi,
I'm trying to figure out a way to configure exim such that a certain user
id can only send mail which is delivered locally. Specifically, I'm
seeing a lot of abuse of CGI scripts to send spam. I'd like to prevent
the userid that CGI scripts run as from sending mail that will leave my
network.

Any suggestions on how I could implement this?

Thanks,
    Rick