What does "This Channel has been closed" mean?

A channel closure is when a network staff member sets a badchan on a particular channel, which in turn closes the channel and disables all access to it. This action is used in extreme cases where a channel has repeatedly violated network policies and rules. It is a last resort, and never done before all other means of resolving the problem have been exhausted.
This FAQ entry's short URL is http://www.darenet.org/faq/12