[CentOS] Signing In Other than Root

Hi;
How do I make it so that it's impossible to SSH into the server directly as
root? That is, make it necessary to SSH in as an ordinary user and then su
to root?
TIA,
Suzie
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.centos.org/pipermail/centos/attachments/20100105/9d567d47/attachment.html>