There are two options for leaving a channel:
Hide: Remain a member of the channel but remove it from your list of channels on the left-hand pane.
Leave: Remove the channel your list of channels and abdicate membership.
To leave a channel, you can also enter the command /part or /leave in the message window.
Every channel has an owner, and the owner of a channel cannot leave a channel until setting someone else as the owner of that channel.