To overcome this you can unblock these or block some other websites and play pranks !
Do The Following :
For eg you want to block www.xyz.com !
* Open the folder C:\WINDOWS\system32\drivers\etc
* There you will find a file named HOSTS
* Click on the file and press SHIFT and now right click on it .
* From the right click menu select Open with .
* Now, select Notepad to open the file from the list !
* Now, in the file under the line 127.0.0.1 localhost add another line as 127.0.0.2 www.xyz.com.
* Now, File>>Save !
Now, open your web browser and try openning www.xyz.com , it will not load !
To unblock sites just do the opposite !