Go to this location..
"C:\WINDOWS\system32\drivers\etc"

There you'll find a file by the name "hosts". Open this file using notepad.
The last line of this file will look like this..

"127.0.0.1 localhost"

Now below this line add this..
"127.0.0.1 xxxxxxxx"
(Where xxxxxxxx is the website that you want to block on your computer.)

Now save the file.. If having difficulty in saving the file change the properties of the file. Its default might be set as "Read-Only"

P.S. do not write http://_______ before website name