If you are using Windows Operating System and did not use Proxy to connect to the internet, then search for the 'hosts' file or if you use Windows XP and use Windows XP standard installation, you can find the file at C:WINDOWSsystem32driversetchosts

These file used as a mapping site URL to your localhost IP.

You can use this trick to block sites access and also blocking ads sites, so you can surf the internet faster.

For example, if you want to block library.monx007.com site, just add the codes below at hosts file:

127.0.0.1 library.monx007.com


Another example:

127.0.0.1 us.a1.yimg.com

127.0.0.1 www.doubleclick.net

127.0.0.1 monx007.com

127.0.0.1 games.monx007.com

Source:
How To Block Sites Access Manually