Easiest way is to put a FAKE Proxy server into Internet Explorer and Firefox then list the 1 site as an exception.
A Proxy server, if you think about the word "proxy" is any server which goes out and retrieves the websites on your behalf, like telling someone else to do your bidding.
Most proxy servers also have "filtering and logging" capability which is why they are used by Schools and large companies to keep kids and employees from accessing inappropriate sites.
By putting a fake, non-existent proxy server into your web browser, you are telling it that ALL internet traffic must go thru this server except for the 1 site you tell it is an exception.
Since the server doesn't really exist.. the websites will never actually load..they will show "page can not be displayed"
This is an OLD Computer Tech trick.. to keep employees off the inernet and restrict them to only internal sites.
The easy way around this is for someone to simply go in and delete the proxy settings..thus restoring the browser back to full functionality.
A way around THAT..is to use a registry hack to lock out users from being able to access the Internet Options.
If you are on a domain.. then all this can be set from within Group Policy...otherwise you need to individually lock out each computer.
It depends on how computer savvy your users are.
The settings are under Control Panel | Internet Options | Connection (tab) | LAN Settings
Put a checkmark next to "use a proxy server" and click on "bypass proxy server for local addresses"
then click on Advanced and specify the addresses you want to bypass separating each entry with a semicolon ;
example: www.myhomepage.com;www.microsoft.com;www.gmail.google.com;
You can list as many as you want.
If you have a long list, you may want to write it down in Notepad as a TXT file and then copy/paste it into IE/Firefox.
This also allows you to replace the settings if they go missing.