I have done this for short stretches, 2-4 weeks at a time, by giving my cable modem to a friend with strict orders not to give it back until a specified date.
It made a huge difference in my productivity and my overall well-being when I didn't have internet access.
When I sat down at the computer, I got right to work, since there was nothing else to do — no email, HN, reddit, etc. (I'm not a gamer, at all; that might be a distraction for others.)
When I was burned out working for the day, I turned the computer off and did other things like reading and relaxing. No aimless web surfing, useless forum opining, etc.
When something came up that I needed to research or do on the web, I wrote it down in a Word doc, and once or twice a week I'd go to McDonald's and use their wifi for an hour or two, knocking out all the specific web tasks on my list.
At present, I am a programmer and I work remotely on software that runs in the cloud, so it's very nearly impossible for me to go without internet at home.
That's a bummer, because my past experience tells me my life and my productivity are both immensely better without it.
I urge you to give it a try. If you don't want to cancel it right away, consider giving your modem to somebody to keep for a month. (One time I mailed it to my mom, and then she wouldn't send it back, for my own good, she said — I had to buy a new one — so... don't trust your mother.)