There are many pages how to disable ipv6 under Debian, but most of them was not working for me.
The most easiest work well:
Modify /etc/default/grub:
GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1"
Don't forget to run update-grub after change (and reboot).
Then if you run
$ ip a
you should not see any IPv6 addresses...