sonicwall to block msn messenger file transfer

Does anyone know how to configure sonicwall to block msn messenger file transfers ?

Ports 6891-6900 enable File send

we only open port 1863 in our ISA server.
we use a sniffer but there are no ports other than 1863 that msn is using.

I think in port 1863 MSN messenger connects to the Microsoft's Server which coordinates everything and transfers the text of each chat session. For file transfers, MSN messenger connects directly to the other person to initiate the file transfer, opening port 6891 for the first transfer, 6892 for the second etc.

so how can i block file transfer between msn users in the same network

If you block connections from these ports you should be ok (I've tested it in the past). But in case a firewall is not intercepted between the hosts that you wish to control, you will have to block connections from these ports on each computer locally.