/sbin/ipchains -A input --source --destination 64.4.12.31/24 -j DENY
BillNe, be weary, MSN will port hop until it finds an open port, it isn't limited to a specific port.
Yes, MSN messenger talks to hotmail.com, more specifically:
msgr-csX.msgr.hotmail.com where X is a number.
Name: msgr-cs1.msgr.hotmail.com
Address: 64.4.12.30
Name: msgr-cs2.msgr.hotmail.com
Address: 64.4.12.31
I think that should do what you want. You could always add it to /etc/rc.d/rc.local if you don't want to template it. If you really want to get advanced I recommend reading the Ipchains How-To, there are a plethora of options available.
Hope this helped,
Nathan