|If there all Windows computers, modify the hosts file on one machine with the websites names resolving to an ambigious address, like 127.0.0.1, then copy that hosts file to the other computers. You can do it during lunch time, after hours, or even across the network through the administrative shares.|
You can also buy a better router that allows for more blacklisting sites, like a Sonicwall or Watchguard.
If you get a server and set it up as a domain controller for the other computers, you can setup 127.0.0.1 for the websites you want to block through DNS manager.
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