[exim] Deny relaying for only one local user

Hello list!

I want to forbid relaying of messages for only one user, so he can only
send messages to local users, not to the world.

Unfortunately we are still using Exim3 (on a debian sarge machine) as
MTA here and I did not find any hints for implementing of an adequate
rule/router/transport/whatsoever...

I'm quite sure that must be a possible. Has anyone an idea?

Thanks,
Robin