Check the favorites folder, 3rd icon on toolbar, a folder with #+ written on it. If that contains the unwanted channel, delete it or edit it not to autojoin anymore.
Also check Options (alt-o) -> Connect->Options -> button Perform... if that contains anything for any networks.
Lastly, go back to remotes (press alt-r twice), then go to Menu View and see if that contains multiple files. It shouldn't matter since you used /!remote off but you can check anyways