Pretty sure you can't do that. You can change which groups can view it. I think priorities only work by protocal, not proto + end point. Even if you could map the ip of msn, and allow communications with that site to be limited, they use a lot of load balancing.
Also, the limits wouldn't even come into play until your "qos" % (start prioritizing when usage at bandwidth at X %), and then MSN web pages would not make a drastic effect anyways (web pages are pretty small).
To block out messenger (which it sounds like you are talking about...the Instant messenger) you would use the intrusion detection rules, go to the chat and messaging category, and then disable MSN. From what i just read in another post in this forum ( hi ) there may be some new issues. The easiest way to block this type of program (4 bagilliopn different program and port signatures) is to use the intrusion detection (SNORT) rules to block it rather than on packet filters (take the free online class from astaro...and ask a lot of questions after you screw around in astaro for awhile).
The next best bet is to block Micrsoft passport and just not allow Passport to run...may have ill effects if people are still using hotmail (ack), ms money or a few FORD websites.
packet: tcp source (you) 1:65535 to destinmation (passport server) 1863
