Here is a quick how to block a specific IP address with the IPtables Linux firewall.
Open the IPtables script :
vi /etc/sysconfig/iptables
To block INBOUND IP address, add the following :
iptables -A INPUT -s 0.0.0.0 -j DROP
To block OUTBOUND IP address :
iptables -A OUTPUT -p tcp -d 0.0.0.0 -j DROP
(replace 0.0.0.0 with the IP you want to block)