I'm not at the point where I need to disconnect my laptop, but during exams last week and the week before that I used this user stylesheet in Firefox:

    @namespace url(http://www.w3.org/1999/xhtml);
    
    @-moz-document domain(twitter.com), url-prefix(https://www.google.com/reader/), domain(news.ycombinator.com) {
      body {
        display: none;
      }
    }

It would have been easy to turn off if I had to (I didn't), but it worked great when I'd idly click a bookmark to one of these sites to be given a blank screen.