OS 10.5 - Website Blocking and page re-direction.

Hi

We are trying to block our staff using various websites like, myspace, facebook and internet radio. We have used terminal on everyone's mac to isolate each website, so when they try and access it, you get either 'Safari can't connect to the Server' page or 'Server not found' page in Firefox. Is there anyway of inserting another line of code into terminal (or any other way) to redirect to a customised web page that we produce, with our logo on stated 'Restricted Access'?

Any suggestions would be gratefully received.

Mark