BitComet uses random ports for outgoing connections. There is no specific method for blocking just BitComet. And it wouldn’t do you much good, since once the users figured that out they could very well switch to another BitTorrent client. And there are dozens of them.
You need a method to block peer-to-peer traffic, period. There are specific tools which some of the ISPs use to this end. I’m not aware which are the names of the tools, since I’ve never been interested in this very much.
One simple and fast solution which would partially solve your issue, would be to block in your firewall, all incoming ports to your network, except those on which you have servers listening and register those which are open for the IPs of the servers you are using (if you have any servers behind NAT). But that’s something you should have done already anyway, if I come to think about it.
Or is your network lying there with all the ports unblocked, like a house with all the doors and windows open, all the lights on and nobody home? 
This would render all their clients in “no listening” mode and would greatly reduce the speed at which they can download, thus releasing some of the stress on your network. However, if they switch to a client which support NAT traversal or choose very well seeded tasks, this wouldn’t pose too much of a obstacle.
Another possible solution would be a proxy firewall, which filters traffic on a per-protocol basis. In this you could allow only certain protocols to run, while denying all other connections be they outbound or inbound.
As I said in the beginning there are tools that some ISPs use for traffic shaping which, I believe, use deep-packet-inspection. You would need a firewall-like tool which can and will perform checks on the payload and upper-layer headers inside the packets going out from your network. They usually inspect the content of all packets and drop those who don’t meet the criteria defined in their filters (where you can put BitTorrent protocol headers or any other P2P protocol patterns, for that matter).
Since you said nothing about your network’s topology, there isn’t much else that comes to mind now.
Here are some links you might want to check:
http://forum.imfirewall.us/forum/default.aspx?g=posts&m=76
http://www.ipp2p.org/
http://www.dslreports.com/forum/remark,17006353
http://www.zeropaid.com
/bbs/showthread.php?t=26680
http://serverfault.com/questions/82055/block-p2p-downloading-in-my-office
http://searchcio-midmarket.techtarget.com/expert/KnowledgebaseAnswer/0,289625,sid183_gci1023730,00.html
http://forums.anandtech.com/showthread.php?t=19746http://serverfault.com/questions/1515/soho-throttle-bittorrent-traffic-from-problem-users