While Freedom isn't Free, this program is.

Freedom is an application that disables wireless and ethernet networking on an Apple computer for up to three hours at a time. Freedom will free you from the distractions of the internet, allowing you time to code, write, or create. At the end of your selected offline period, Freedom re-enables your network, restoring everything as normal.

Freedom enforces freedom; a reboot is the only circumvention of the Freedom time limit you specify. The hassle of rebooting means you're less likely to cheat, and you'll be more productive. Not rebooting is why we bought Apple computers in the first place. When first getting used to Freedom, I suggest using the software for short periods of time.