There are some simple steps to follow, in order to disable ipv6 networking on CentOS. This simple guide is about CentOS 6.4, but I think that it will work on both a little older or next versions of this Operating System.
Step 1: add this rule in /etc/sysctl.conf : net.ipv6.conf.all.disable_ipv6 = 1
Step 2: add this rule in /etc/sysconfig/network: NETWORKING_IPV6=no
Step 3: add this setting for each nic X (X is the corresponding number for each nic) in /etc/sysconfig/network-scripts/ifcfg-ethX: IPV6INIT=”no”
Step 4: disable the ip6tables service : chkconfig ip6tables off
Step 5: reboot