The PIX does not have the ability to perform deep packet inspection to the level needed to catch skype, only ASA has this ability. But on PIX you could try to block UDP port 1247 and TCP ports 2940-3000 but if these ports are blocked, then skype uses 443 to tunnel out of the network to talk to the SN's (supernodes). Unless you plan on blocking all SSL connections, this is really not a feasible fix.