In fact it might prove very difficult to block Skype without blocking everything else as well.
Skype uses various methods of connecting via the internet with its basic mode using solely the ports assigned to web browsers. Under this situation Skype will still work but with reduced quality or features.
To quote the Skype technical user guide on Firewalls ..
There are four options for Skype to work:
- Ideally, outgoing TCP connections to all ports (1.65535) should be opened. This option results in Skype working most reliably. This is only necessary for your Skype connection to be able to connect to the Skype network and will not make your network any less secure.
- If the above is not possible, open up outgoing TCP connections to port 443. This will only work if you are using Skype version 0.97 and above.
- If the above does not solve the problem, open up outgoing TCP connections to port 80. Some firewalls restrict traffic to port 80 to HTTP protocol, and in this case Skype can not use it since Skype does not use HTTP. In some firewalls it is possible to open up all traffic to port 80, not just HTTP, and in this case Skype will work.
- If the above is not possible, Skype versions 0.97 and above can use a HTTPS/SSL proxy. In order to do that, you have to configure the proxy address in Internet Explorer options. Skype will then be able to use it as well.
So option 3 is basic mode of operation with port 80 (which is the port necessary for the web browser to work) and port 443 (used for secure HTTPS web connection). So to block everything except HTTP/S protocol will block Skype but will also cripple the internet connection and service provided.
Most ISPs will use proxy servers to FILTER out content and not firewalls to block ports.
Bottom line. Nothing is technically impossible but to block Skype without impacting on the quality of internet service provided is not possible.