Freedom

Freedom is an application that disables networking on an Apple computer for up to eight 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 enjoy enhanced productivity.