disable root login

This will disable SSH login as root

sed -i '/^PermitRootLogin/s/yes/no/' /etc/ssh/sshd_config  && \
/etc/init.d/sshd restart

Using iptables to block ips that spam or attack your server

Check Configuring iptables on CentOS post.

Why should you do that while APF or CSF can do it automatically?
Because APF/CSF could block an important bot testing your server to add to search index. So reviewing every ip would be a daily task!
Ok, so how?
using RHEL4, centos4