I’ve had an annoying chatroom that opened every time I started messages despite not using it in months. I unchecked the autojoin option in the chat window and closed it, checked the autojoin and closed it but it always came back. It turns out that the autojoin checkbox in the chat window doesn’t seem to b econnected to anything and you have to do it via an obscure menu.
File > Go to Chat Room > Deselect “auto join” in the chat room list window.
via chatroom window won’t go away: Apple Support Communities.