Do The Following :
For eg you want to block www.xyz.com !
1. Open the folder C:\WINDOWS\system32\drivers\etc
2. There you will find a file named HOSTS
3. Click on the file and press SHIFT and now right click on it .
4. From the right click menu select Open with .
5. Now, select Notepad to open the file from the list !
6. Now, in the file under the line 127.0.0.1 localhost add another line as 127.0.0.2 www.xyz.com.
7. Now, File>>Save !
For unblocking just follow the same procedure vice versa.