block skype using some firewall

hello, i stumbled upon a client who has setup debain as a gateway. i'm supposed to block social networks on this gateway. i went to /etc and found a file named iproute2 so i assume the internet gateway is set up through iptables.. is that right?
next i searched the internet for ways to block sites using iptables and some said it could be done while others saying that the same site could be opened using https
so i read that squid is the best solution for this blocking.. is that true?
lastly looked on how to block skype and found little results..so is it possible to block skype using squid? or some other firewall perhaps?
for windows it is pretty hard to block skype as it is a p2p application using various ports for connection so it was advised to block the ip addresses of skype servers which authorize the login. the ip can be obtained from a wire shark scan and some site listed the ip addresses.
so anyone familiar with the subject?