Check this link:
http://support.skype.com/index.php?_a=knowledgebase&_j=questiondetails&_i=148
It is very diffcult to lock down by port as it uses a variety of TCP ports as well as 80 and 443 so I think what you could do is deny traffic from inside to the Network IP for skype.com and skype.net?
Are you are asking for the syntax to create the ACL?
HTH, please rate if useful
