Find it hard to concentrate this week. Going to give this a try (after I have posted this!).
SelfControl is a free and open-source application for Mac that lets you block your own access to distracting websites, your mail servers, or anything else on the Internet. Just set a period of time to block for, add sites to your blacklist, and click "Start." Until that timer expires, you will be unable to access those sites--even if you restart your computer or delete the application.