Meet SelfControl, an app by Steve Lambert that I've been using all the fucking time nowadays.

SelfControl is an OS X application which blocks access to incoming and/or outgoing mail servers and websites for a predetermined period of time. For example, you could block access to your email, facebook, and twitter for 90 minutes, but still have access to the rest of the web. Once started, it can not be undone by the application, by deleting the application, or by restarting the computer – you must wait for the timer to run out.