esta página parece informativa. parece que funciona se for executado como root. Eu não sinto como desativá-lo permanentemente, então eu não posso confirmar o comando sysctl.
root@Shambhala:~# ifconfig
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
inet 127.0.0.1 netmask 0xff000000
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::21b:63ff:fea5:de87%en0 prefixlen 64 tentative scopeid 0x4
media: autoselect status: inactive
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet 172.16.11.98 netmask 0xffffff00 broadcast 172.16.11.255
inet6 fe80::21c:b3ff:fe77:c434%en1 prefixlen 64 tentative scopeid 0x5
media: autoselect status: active
supported media: autoselect
vboxnet0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::276:62ff:fe00:0%vboxnet0 prefixlen 64 scopeid 0x7
ether 00:76:62:00:00:00
root@Shambhala:~# ip6 -x
root@Shambhala:~# ifconfig
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
inet 127.0.0.1 netmask 0xff000000
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
media: autoselect status: inactive
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet 172.16.11.98 netmask 0xffffff00 broadcast 172.16.11.255
media: autoselect status: active
supported media: autoselect
vboxnet0: flags=8842<BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
ether 00:76:62:00:00:00
parece que o ipv6 permanece ativado no dispositivo de loopback ...