Here is a simple trick we will Block a webpage without any software !!!!
so easy and simple! just follow these steps

Lets block Facebook!!!

1)  Open  Notepad its important to run it as Administrator 
Start -> All Programs -> Accessories -> Notepad (Run as administrator)

 

2) Write "127.0.0.1 www.facebook.com " into the notepad (If you want to block more sites just type
127.0.0.1 www.anywebpage.com in another line)

 

(this will block Facebook , yahoo and Google but you can write any site you want)

3) Save it in to the below address as "hosts"
C:\Windows\System32\drivers\etc

select "all files *.* "  from  Save as type part (as in below picture)

(Do same as show in red parts of picture )

Confirm the dialog box 

its done now try to open facebook!!!!! 
you  can't ! its blocked 

to Unblock :
It is same as blocking!
but in this time save a blank file or delete the site you want to unblock it from the Hosts file