Depends on which ver. of IOS you have. I've used several 28xx and 38xx w/ advanced security IOS using IPS that will allow you to tune the IPS signature to drop the packet if it matches a particular application (such as Kazaa or MSN Messenger).
The most simplistic way of acheiving this is an ACL.
access-list 101 deny tcp host 20.20.30.x any eq 554
This allows you to be more specific whereas the IPS example is more global.
