Currently, when using external authentication, there is an option "Auto-add users into FishEye" for JIRA/Crowd AND LDAP, that adds a user automatically to Fisheye upon successfully authentication when logging in. This user will also be added to Crucible at the same time. However, this leads to a problem if that user does not need access to Crucible.
The workaround is to manually go through the user list and disable Crucible for those users who don't need access to it. To prevent this, a property/checkbox called "Do not automatically add users to Crucible" would be handy when using external authentication.