In our write up of distraction free studying we mentioned SelfControl.
It's a beautiful little app for Mac that allows you to block what any website for a period of time. The brilliance in the app comes from its simplicity. Slide the slider for how long you want. Enter your black list of websites you want blocked and click Start.
There is nothing you can do to reverse the block. It will survive logging out and logging back in, it will survive you changing user, and it will event survive a restart. It is the true master of Self Control.