I could imagine a solution of having two computers, preferably in two rooms -- one for fun, another for work. The idea is that you are completely forbidden to have any fun on the working computers (and there are a few programs to make sure you don't visit funny websites, etc). Any time you could go from one computer to another, it's just inconvenient and you cannot pretend to work then you are not.

More meta: with enough money, this kind of problem can be solved easily. Most straightforward solution would be to pay some strong guy to guard you the whole day and physically prevent you from doing any of the typical procrastination.