Just follow these steps:

1.Browse for C:\Windows\system32\drivers\etc.

2.Search for the file named "hosts" and open it in notepad.

3.Then you will see a line "127.0.0.1 localhost".

4.Just under this line add "127.0.0.2 www.sitenameuwanttoblock.com" without quotes.

5.Change the www.sitenameuwanttoblock.com url with the site's url you want to block and you are done!!!!

Now that site will not be accessible.

Comment for any problem or help and support here.