We can disable the IPV6 in Linux systems, the following steps are should be disabled to all major Linux distributions including Ubuntu, CentOS, Fedora, RHEL, Debian, Linux Mint.

You have to open the sysctl.conf file using any text editor (vi, vim, nano), 

# vim /etc/sysctl.conf

at end of the line copy and paste the following lines in sysctl.conf file

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

Then, save and activate the configuraiton.
 
# sysctl -p

You will see the below output,


net.ipv4.ip_forward = 1

net.ipv6.conf.all.disable_ipv6 = 1

net.ipv6.conf.default.disable_ipv6 = 1

net.ipv6.conf.lo.disable_ipv6 = 1


Then, you can check whether it's disabled or not,
 
# cat /proc/sys/net/ipv6/conf/all/disable_ipv6
1