First: you should always inform, on which system you are: joomla? cb? ... If not we cannot help
You have to set in Joomla "Allow user registration" to "No"
and in CB configuration=>Registration "Allow user registration" to "Yes, independently of global site setting".
I´m having the same problem. I´m also working with Joomla 1.7.3 and CB 1.7.1.
Did you find a solution? It would be nice to get the CB login instead of the Joomla login or maybe a custom page.
My configuration is ok: Joomla user registration is off and CB registration is set to "Yes, independently of global site setting".