It was the middle of a boss-fight and the only three people with moderation prvilages present were the main tank, main healer, and myself. I kicked the offenders quickly to prevent further disruption to the encounter, but they came back, so I banned them. They came back again (I hate it when they're so persistent...), and being still in the boss encounter, all I could do was set a password.
After a short time, I took the password off, foolishly hopeful, and we got flooded again during another encounter, so I turned the password back on quickly as a stop-gap measure.
At the end, I must confess that I forgot to disable the password on the channel, it wasn't intentionally left on. I've removed it now (it was still on when I checked a moment ago), sorry to all who couldn't get in!
I'd have tried to fiddle with the Auto-Voice thing (I'm not all that familiar with it yet), but as I said, we were mid-encounter and I could only tab for a few seconds at a time.
I have never intended to close people out (save griefers, obviously), and if I have to do the password thing again as a stop-gap, then please, for the the love of all that is good, someone poke me at the end to take the password off again!