The Problem : The standard Joomla 1.5 stable has no means to allow or require admin (or other) approval / verification before a user is allowed to login to a site - that is - after they get their verification email, all one has to do is click and then login to the site. It would be just absolutely fantastic if the admin - besides being notified of a new user - could actually approve or deny an application for registration. See at Joomla Forum
JoomlaXi Solution : We have developed a plugin, called Admin Approval Plugin. Once it is installed on your site, the administrator of website can selectively approve the joining request.
How it works :