1. Edit /etc/sysctl.conf  and add the following line

net.ipv6.conf.all.disable_ipv6 = 1

2. Edit /etc/sysconfig/network and add the following line

NETWORKING_IPV6=no

3. Edit /etc/sysconfig/network-scripts/ifcfg-eth0 and add the following line

IPV6INIT=no

4. Stop iptables6 service

service iptables6 stop

5. Disable iptables6 service

chkconfig ip6tables --level 345 off

6. Reboot the server

reboot

Done IPV6 is disabled on your CentOS 6.X server.