You can somehow change your CD key for warband in the windows registry files, I'm guessing they somehow do it during when they're in the server which causes the auto kick messages to appear?
No, the UID isn't set in the registry base.
However, here is found the CD Key which determines what the UID will be,
but if your CD Key is incorrect, you won't have a multiplayer identity therefore you won't be able to join a multiplayer server.