The private chat channel is being generated by the notifier app
. It sends website requests to your Windows browser instead of the in-game browser. If you don't want this feature, open the notifier (system tray near the clock) to the URL Helper page and turn off both checkboxes. This will stop the channel from being created, but it won't delete the channel from your characters. You can use /leavechannel or the Chat tab in the utility to quickly remove it as you did before.