The PIX can only block traffic either by IP address or port and never on an application level. Blocking Instant Messaging programs is a bit difficult and tricky. The type of application that you are trying to block has the ability to dynamically change ports which makes blocking them difficult. The best way of blocking IM would be to search for the IM server's IP addresses and block them from the inside network via access-list or conduits. However, like the ports used by the application, the IP addresses also changes every now and then.
