How to block websites temporarily?

The tool I'd recommend is Get-Shit-Done. It edits your host file so that you cannot access blocked domains using any browser. However, it does not have a scheduler (although you could use cron for that).

Stay Focusd is Chrome Extension that does something similar.

I personally use a secondary profile in Chrome, with the Whitelist extension, so that it blocks all websites, other than absolutely necessary ones

Open Terminal (CTRL+ALT+T).

Type sudo -i in the input field. Check the Run in terminal option. Finally click Run button.

Type your password if necessary and press enter. Then do the following command.

gedit /etc/hosts

You will get gedit Text Editor window.

For example, if we need to block Facebook just add the following lines just after 127.0.0.1 localhost.

0.0.0.1 facebook.com    
0.0.0.1 www.facebook.com

By doing this, it will block the site in all browsers including Google Chrome, Chromium, Mozilla etc.... In fact, you can't access the website at all by any means.

That's it. When you now open www.facebook.com or facebook.com, you cannot access it. To enable back Facebook, remove the lines we added from the file /etc/hosts or as Eliah Kagan said you can add a # character before the 0.0.0.1 text we added in /etc/hosts to make it a comment and to block it again just remove # character from the beginning of the line.

Example

Blocking :

0.0.0.1 facebook.com

Enabling It Back by making the line a comment :

#0.0.0.1 facebook.com

Source

If you use Firefox, the LeechBlock add-on looks like it might do what you wish.