There are many ways to do this type of thing. One way of doing this is to have an insert query to put the user into a database table and specify that this user is not active until a certain value is changed.
When the user signs up, maybe an email is sent to the administrator that notifies him someone new has signed up. He then goes into the database and reviews the information sent by the user. If it is ok, he makes the user valid and notifies the person that his account is now valid.