How to block an IP on your server?

I was for a long time trying to block some ip on my server...
People are sad with me and want to connect to it even if they have no
rigth to do it.
They try several login like bar, computer... Bad people :op.
So I found that and it works well on my redhat :
http://www.cyberciti.biz/faq/how-do-i-block-an-ip-on-my-linux-server/

The main idea is :
to block for everything : # iptables -A INPUT -s 65.55.44.100 -j DROP
to block for a port : # iptables -A INPUT -s 65.55.44.100 -p tcp
--destination-port 25 -j DROP

To see what are the IP already blocked, you can use iptables -L

Cool, isn't it ?