I know this post is a bit older, but is there a better solution to the problem.
I also want to activate new members manually, but the user should receive a mail with an activation code, so I can be sure that he has registered with his e-mail address.
The solution with the mentioned plugin doesn’t work either, because when the user activates his account, he can already log in on the site, no matter if he has already been activated by the administrator or not.