Many people think that blocking websites is a hard task, it can be very easy.
You don't need to install anything and it takes 1 or 2 minutes.
All you have to do is go to this directory:
Open the file hosts file with notepad
(if you are in windows 7 right click the hosts file and click Run as administrator)
add this line to the hosts file:
and you are done!
If it doesn't work try adding the www. Preffix.