To check if IPv6 is enabled or disabled

$ cat /proc/sys/net/ipv6/conf/all/disable_ipv6 

0 means it’s enabled and 1 is disabled

To disable IPv6:

$ su - 
# nano /etc/sysctl.conf 

and add these lines to sysctl.conf file:

#disable ipv6 by adding any of the below 
net.ipv6.conf.all.disable_ipv6 = 1 
net.ipv6.conf.default.disable_ipv6 = 1 
net.ipv6.conf.lo.disable_ipv6 = 1 

net.ipv6.conf.wlan0.disable_ipv6 = 1 
net.ipv6.conf.eth0.disable_ipv6 = 1 

Save sysctl.conf file with new config, then reboot your system

References

https://askubuntu.com/questions/316492/disabling-ipv6-on-a-single-interface