SelfControl is a free and open-source application for macOS 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.
SelfControl is now available in 12 languages thanks to the fine translators credited here.
SelfControl is free software under the GPL. See this file for more details.