If you want to enforce the opposite of what is described in Section 18.6.5, i.e. be able to approve each and every user before he is allowed to
register, then you should have a look at the Approve Membership module. See Section 8.3.4 and Authorize accounts for more details.
If you want users to be registered directly, without confirmation mails, see Section 18.6.5. If you want to disable registration, see Section 18.6.4.