How to block someone's IP

To block incoming IP, use this command:

   iptables -A INPUT -s IP -j DROP

For example:

   iptables -A INPUT -s 123.123.123.123 -j DROP

To save the rules, run:

On Centos)

  /etc/init.d/iptables save

On Debian)

Please read this link http://www.debian-administration.org/articles/445