Disallowing Users to Login

If you’re looking to prevent certain Users from logging in, you can use this hack as a guideline. This hack hooks directly into the processes in /wp-login.php, which means it will work for logins from directly at /wp-login.php, and from things like the s2Member Pro Login Widget.

In this hack, we’re redirecting all Users (except Administrators) with the Custom Capability deactivated to the homepage.

ID, 'access_s2member_ccap_deactivated') || current_user_can('administrator'))
		return;
		
	wp_redirect(site_url('/'));
	exit();
}

This is now irrelevant

See this plugin to achieve this: http://wordpress.org/plugins/member-approval/