It's dumb, but in the past when I felt I was wasting too much time on the internet, I added some entries like this to my /etc/hosts file:

  127.0.0.1	reddit.com
  127.0.0.1	www.reddit.com
  127.0.0.1	m.reddit.com
  127.0.0.1	old.reddit.com

This is such a minor impediment, but I found in practice that it greatly reduced the time I was just fooling around websurfing. I don't think I ever disabled Hacker News, but I've rarely been able to blow multiple hours here, either.