Below is a list of the main Instant Messenger applications (including ports and destinations) for the denial of use via policy based rules.

Please note : With creating policy based rules the following rules will be required,

1. Destination any with a service port of the below ports (excluding http and https)
2. Destination of the below with a service port of http/https.

Skype

Skype can be extremely difficult to block due to the way in which the Skype protocol functions.

The Skype protocol is designed by default to circumvent conventional firewall blocking methods. It will attempt to connect via ephemeral ports UDP/TCP 1024-65535, if connection via any ports within these ranges fail it will attempt to connect out using HTTP or HTTPS.

The only way to block this traffic is via payload inspection, which is typically performed via IDS/IPS engines.
Due to the initial server / client message exchange being non-SSL, denying payloads which include the value 0x170310000 will prevent the establishment of Skype.