lordon Just remove the website role for the hostname. E.g if your hostname for that server is host1.domain.com, go to websites and delete it. That will default to users having email settings related to their domain name and not the server hostname.
Same thing when they click the login to webmail, if you want the default roundcube to be mail.theirdomain.com, then remove the webmail website on the control panel. But then that means they will not access webmail / roundcube if propagation for their domain is not complete.