you need to customize JWChat, and as you enter the domain name you can (and should) as Jason did say also hard-code a user and the password. I think you do not really care who is connecting but as it is easy possible to use the conference and search service you may dislike JWChat.
There’‘s also MUCkl available which is a group chat, there you just enter a username of your choice. The problems are the same, both products come from http://zeank.in-berlin.de/ and both use JEP-0124 (HTTP Binding) also from the same author. As all are open source it should be possible to modify the JEP-0124 code to disable search services and restrict group chat to one room. I don’'t like this option.
Some days ago one did ask how to limit access to a component, so this was a similar question.
@Gato: It would be nice if Wifi could support ACL’'s for components, plugins and users.