What also works is Freedom (mac only): http://www.ibiblio.org/fred/freedom/
It disables ethernet for a specific period of time. You can only turn it off by rebooting. It's drastic (but I need drastic).
Another option, for those using an apple Airport Base Station, is to activate the Timed Access control for your computer's MAC address. I, for instance, cut off my net access between 9am and 5pm on the weekend.
Perhaps we can even make it work with only one computerL
Dual boot, giving one partition nothing but internet access, and the other partition everything except internet access?
Not the same thing. The idea is to physically move your body far enough away from one or the other. You can only do one at a time. Your reptilian brain must sense that something is wrong (or right). Flicking a switch while still sitting on your butt isn't enough.
Yes. But if you have no wireless and a short cable and therefore must carry your laptop across the room to connect to the net - it might just work.