Sorry, but if you need to continue to have access to the workstations via ARD, as you stated in your other thread on the issue, there is no way to block Internet access on the workstations. If you can't affect the router, there's nothing you can do to block connection to the Internet, at least nothing I can find that couldnt' easily be undone and/or can be done on the fly (that is, without making alterations to preference files and restarting the systems).
As I suggested in my most recent post in your other thread, set up a different user account on each workstation with the inappropriate applications disabled and then log the workstations into that account for sessions where you do not want the students to be able to use those apps.
Otherwise, I can't see that there is anything practical that you can do since a management solution such as LANSchool or Mac OS X Server or other non-free solution doesn't appear to be an option for you.
Message was edited by: varjak paw
Possibility 1: Set a Proxy server, but put the lab in the exclusion list. That should stop anything that honors the proxy server settings.
Possibility 2: Create your own DNS server and configure it so that it does not translate any domain names outside the lab. Then configure all the Macs to use the lab DNS server.
It might even be possible to just put in a bogus DNS address as the DNS server on each of the Mac (10.100.200.1 - as 10.*.*.* addresses are non-routing this should not go anywhere beyond the router). In theory Bonjour should still work for talking to in lab stuff (guessing here).
The easiest thing to do is use ARD to set the proxies to a non-existant proxy server (127.0.0.1) when you want to disable www.
There is a template in Send UNIX Command>Templates>Network Setup>Web Proxy
Replace the last line of the template with these two and send to clients (test one Mac first):
networksetup -setwebproxy "Wi-Fi" 127.0.0.1 8080 Off
networksetup -setsecurewebproxy "Wi-Fi" 127.0.0.1 8080 Off
Replace "Wi-Fi" with whatever their active Network conenction is named in System Prefs>Network, as sometimes it has a custom name if Wi-Fi has been retrofitted.
To turn internet ON, repeat the above with commands to disable proxies..
networksetup -setsecurewebproxystate "Wi-Fi" off
networksetup -setwebproxystate "Wi-Fi" off
You can save these Tasks in the sidebar or in the Send UNIX Command>Templates menu and name them "Internet OFF" & "Internet ON".
Then in future, you can just Right-click and choose Run Task.