One quick and dirty fix that I used for a site that refused to work in Firefox and so had to be viewed through IE.
Connections Tab, Click Lan Settings
Under proxy server fill in some false information
In the box 'Do not use Proxy Server for addresses beginnig with'
Put in the address of the site you want to be able to access.
If they try and go anywhere else the browser wont be able to contact the non-existant proxy and so will fail.