If you want to block a website on your computer then go to "C:\WINDOWS\system32\drivers\etc " Then open the file hosts with the notepad. Then under this line "127.0.0.1       localhost"
write 127.0.0.1     and your desired website which you want to block. example -

127.0.0.1      www.facebook.com  ; if you wish to block facebook on your computer.
127.0.0.1       www.yahoo.com    ; if you wish to block yahoo on your computer.
sometimes it needs a restart of computer to activate blocking website.