Ok. First off all. Remember. You have four network interfaces.
You need to know from which network users are trying to communicate to skype. This is where you need to block it. So first of all you need to test. I am going to give you more or less a setup just to show you what I mean.
Let say you have got the internet connected trough a network on the first interface(0) with the subnet 10.148.1.0/24. Then you have got an internal network on where all your PC's work on a subnet on interface(1) with lets say the subnet is 10.195.1.0/24. Ok this means that you will have to block the original request to skype. So if a person opens skype with his IP 10.195.1.25, the firewall will not allow the request trough.
I hope you understand what I mean in blocking the packets.
Ok so to get the IP and the port information on your firewall VIA ASDM.
Open ASDM, login to the firewall. Once logged in, click on Monitoring button at the top of the horizontal toolbar. Now click on the Logging button at the left vertical toolbar, then leave the settings default and click View...
Now you will see your firewall's log in real time. You will see there is a place where you can filter. Filter By:. In the box type in your PC's IP for instance.
Now close all your other communication on the network and try to open skype. It should give you the source and destination IP's. Write down the Destination, this is skype's IP address which is used for communication.
Now that you have the IP you can close the real-time Log viewer.
Click on Configuration button at the top horizontal toolbar.
Now click on Security Policy button on the left vertical toolbar.
Now you can see all your interfaces. So lets create the rule then. Remeber in this conversasion you are on interface1 so I am going to use that as an example. It may be different on your setup.
So your interfaces will be in order. Int0 will be at the complete top. You will see its a line with some name in it which you can expand or collaps and underneath it is the rules. So lets say you need to block skype now on interface1.
Click on the text of interface1. At the top of the Access Rules click Add.
Interface : Ensure you use the correct interface
Action : Deny
Source : 10.195.1.0/24 (this will block everyone on this subnet)
Destination : Skypes IP which you determined with the log
Service : this is the port. But I would recommend to leave it on IP as this will block anything to skype. It does not help to specify the port since users can change the skype ports in there settings and bypass the firewall.
Description : Type a description for yourself to understand the rule in the future.
Click OK. At the bottom of the rules. click Apply. Test(while testing view the log again to see if it goes trough or if it gets blocked). If everything is working, remember to click save on the top horizontal toolbar. If you do not save, after a reboot on the FW you will lose that config.
This method might differ a bit depending on the IOS loaded on the firewall.
Please let me know if this have helped and if you are still strugeling.
so how would you do that exactly????