Seconded. I wrote a little python script [1] that I use for this that additionally disables my network adapters during work time.
This is going to sound incredibly lazy, but I've found that a good way to keep myself off the internet during break periods (which often leads to extended breaks) is to play a game of Civilization. One turn per break and you'll be so absorbed you won't even think about HN or Reddit.
I've done it. I save the game right before I'm about to uncover a barbarian hut, then move onto it. If I don't like what the random number generator gives me, I load the saved game and do it again. Over and over again.
See? I've played ONE turn of Civilization, just repeated about 50 times.
Early game, no. Late game, it can take 5 or 10 minutes per turn and you may be capturing more than one city per turn during an invasion, I can see it working.
This is going to sound incredibly lazy, but I've found that a good way to keep myself off the internet during break periods (which often leads to extended breaks) is to play a game of Civilization. One turn per break and you'll be so absorbed you won't even think about HN or Reddit.
[1] http://sha.ddih.org/2010/09/04/pomodoro-py/ (it's rough but it works for me)