edit /etc/ssh/sshd_config

enable the line and change to no
PermitRootLogin no

(You can also edit the file /etc/securetty)

then see the iptables file how we restricted sshd to only listen 
to this subnet


to restart sshd:
kill -HUP `cat /var/run/sshd.pid`

could have restarted it by

/etc/rc.d/init.d/sshd restart