[1] http://freestylemind.com/productivity-experiment-internet-diet
Freedom does this, apparently pretty well. Not CLI, though, apparently.
It checks every minute so you still have a window :)
*/1 0 * * * root ifconfig en0 down
*/1 1 * * * root ifconfig en0 down
*/1 2 * * * root ifconfig en0 down
*/1 3 * * * root ifconfig en0 down
*/1 4 * * * root ifconfig en0 down
*/1 5 * * * root ifconfig en0 down
*/1 6 * * * root ifconfig en0 down
*/1 7 * * * root ifconfig en0 down
*/1 8 * * * root ifconfig en0 down
*/1 9 * * * root ifconfig en0 up
*/1 10 * * * root ifconfig en0 up
*/1 11 * * * root ifconfig en0 up
*/1 12 * * * root ifconfig en0 up
*/1 13 * * * root ifconfig en0 up
*/1 14 * * * root ifconfig en0 up
*/1 15 * * * root ifconfig en0 up
*/1 16 * * * root ifconfig en0 up
*/1 17 * * * root ifconfig en0 up
*/1 18 * * * root ifconfig en0 up
*/1 19 * * * root ifconfig en0 up
*/1 20 * * * root ifconfig en0 down
*/1 21 * * * root ifconfig en0 down
*/1 22 * * * root ifconfig en0 down
*/1 23 * * * root ifconfig en0 downof course you'll also lose all network, not just the internet
I suppose the other way is changing /etc/resolv.conf from
nameserver 8.8.8.8
nameserver 127.0.0.1