You can always use dns to stop IM I found blocking ports not to be effective since IM clients will use port 80 to get out if the standard ports are blocked. I found the best way was to add a dns entry for the server or cluster that hosts the IM then redirect those requests to localhost.
$ORIGIN msgr.hotmail.com.
$TTL 1h
@ IN SOA xxx.xxx.com. hostmaster.xxxxxx.com. (
2002101301 ; Serial
1h ; Refresh
1h ; Retry
1w ; Expire
15m ) ; Minimum TTL
@ IN NS xxx.xxx.com.
@ IN NS xxx.xxx.com.
@ IN A 127.0.0.1
* IN A 127.0.0.1
