I don't think to have understood very well what you say about chatrooms. Chatrooms follow the Joomla access levels for assignment, the same as a Joomla article. The global room is used by default when no chatrooms are selected and has no 'members only' assignment. The only issue that i can think of, is that you have the cache option turned on in the chat advanced option so that the chatrooms are still cached before your changes.
PS: the IP banning feature is already included. Look a bit more carefully to the banning section (screenshot below)
When users exchange messages the IP address is logged in the messages list. If you get spammed by bots you need to know the IP address to ban.