How do I leave the Novice network?

You will not be able to rejoin the channel if you lose your "new adventurer" status. This happens after 168 hours of play time, completing the first arc of the main story questline at level 50, or can be removed manually. Be aware that removing the "new adventurer" status also removes the +20% xp bonus.

/nasoff removes the "new adventurer" status. It can be turned back on with "/nastatus on" if the player has not yet completed the first storyline arc and has been playing for less than 168 hours.

Kicking from the channel does not work, this only removes you for 30 minutes and will add you back in the next time you change zone or log back in after the 30 minutes expires.

Why not mute it instead?

You can "mute" the channel in the log settings. Press the cog next to the chat window, select the chat tab in the "Log Filters", and uncheck the "Novice Network" then apply changes. This allows you to keep your xp bonus without having the chat box filled with messages