hi guys,

One of the student/education network that I manage is basically spliced into 2subnets: (Mac OS X Server with 120 iMacs) (Windows Server 2003 with 45 Win XPPro w/stations)

ADSL2+ Internet connection gateway is via modem/router with both the above Servers doing DHCP/ActiveDirectory/Group Policies/Open Directory/DNS fwd etc. etc. for their respective client machines.
Despite using openDNS (OpenDNS | Providing A Safer And Faster Internet) for Internet filter, I'm still facing huge bandwidth usage from the students...average of ~6GB a day Mon-Fri.
Almost every month, the ISP throttles the speed mid-month due to usage
going over-quota.

I've tried (in a test environment) to completely disallow students access to Windows machines and found that there's not much difference in usage however when I completely disallow students using Macs, I see significant drop in Internet usage.
Although openDNS works as filter for both networks but am not sure if iTunes/streaming etc is being affected by filter hence students on iMacs are being able to spike Internet usage.
Are there any ways to find from Mac X Server how to address this issue? block iTunes? how?

Any suggestions would be much appreciated.