How do you block internet sites?

there are 2 ways to block internet sites.
1. host.cfg

In "Local Disk (C:) > WINDOWS > system32> drivers > etc", you will find a file named "host.cfg". The Hosts file contains the mappings of IP addresses to host names. This file is loaded into memory (cache) at startup, then Windows checks the Hosts file before it queries any DNS servers, which enables it to override addresses in the DNS. This prevents access to the listed sites by redirecting any connection attempts back to the local (your) machine.
You may use "notepad.exe" to edit this Hosts file.
Example - the following entry blocks all files supplied by that DoubleClick Server to the web page you are viewing.

4. Internet filtering products

You also can use internet filtering products to do web filtering. An internet filtering product can be deployed in your network, and it enables you to monitor, filter and block internet activities of all computers from a mirroring port.
More than web filtering, internet filtering product can also block file downloading, block p2p traffic and block messenger.

For internet filtering software, you can try "Aobo web filter"