To Block a certian IP address from hitting your server or a ISP.Please use following commands

######### block a new ip # with iptables ###########
##### replace xxxx with the ip # you wish to block #####

/sbin/iptables -I INPUT 1 -s xxxx -j DROP

####after the above...run this one..to make them stick####
##otherwise..they will clear at reboot or restart of iptables##

iptables-save > /etc/sysconfig/iptables

OR use ipchains

ipchains -I input 1 -s IP-here -j DENY

After the procedure don't forget to restart the ipchains service.

/etc/init.d/ipchains restart or

service ipchains restart