Sorry, when I say basic, I mean in terms of functionality, not method. It seems to be quite easy, though. Go to the Firewall menu first and make an outbound rule to block internet access (see page 69 to 71) for whichever client you want to block, then you can use the schedule to influence when the rule is applied.
Funny how things make more sense with a bit of extra explanation. The list of services look s bit daunting, do you know which one relates to MSN as it's that I want to restrict (appreciate you may not have the list but if I don't ask etc)
Someone asked the opposite question about MSN Messenger yesterday - how to unblock it, so I looked for the port number. It uses TCP port 1863. If you can add that port to your rule, that should do the job.