For a complete blackout just redirect to this site: blackout21.eu/redirect/en_blackout.html
There are many ways to redirect. The best way would be to configure your webserver in a way that it will temporarily redirect (302).
If you're not shure how to do it just ask your support, they should help you.
An other posibility would be to download the site and upload it to your own webserver
This script will block your site completly. It uses their logo and links to savetheinternet.info
The Wordpress Plugin does the same.
This possibility shows information, but can be closed. So the users can still get on the site.
This Plugin blocks the site completly
It was provided by the Pirate Party, but can be customized according to the developer himself.