/help and read every command, and came across the
/chat command. According to the info,
/chat leave General should make me leave the channel, but it doesn't. How can I leave the General chat (but still have access to the other rooms, like LookingForGroup)?
The easiest way is to turn off the subscription to General chat.
On your chat frame, you'll see several tabs, such as 'General', 'Combat' and 'Tell Messages'.
You can right click on these to change the channels and information feeds that each tab is 'Subscribed' to. You'll need to do this on every login for now, but a fix is supposedly 'coming soon'.