The ^ prefix is mentioned in /help /haltdef but the help file isn't very clear. Events prefixed with ^ trigger before the default mirc action(s) take place. One of those actions is displaying the snotice (or whatever other) message received from the server. /halt stops mirc's default actions, but you can't stop what has already happened. This is why /halt on a non-^-prefixed event doesn't do anything: by the time such an event triggers, mirc has already handled the message, ie displayed the default text.
