In DHCP, you could search for MAC of apple devices. Check here for Apple OUI. Ping -a will resolve machine names. Your wireless is wide open? Do you do anything as far as isolation on wireless or have a separate DHCP pool? This could help identify devices. If there are a lot it could get labor intensive, otherwise MAC filtering could be applied on case by case. Also, consider firewall/proxy to block streaming data from known sites
You could always dia-allow ‘itunes.exe’ from running on your devices as well, or being installed. Having an I-pod to listen to music at work is one thing. Installing ITunes on a workstation is another.