When this situation occurs, you can unblock them using the gitlab-rails console directly in the server.
Run the following:
$ gitlab-rails console production
And then inside the console you can find the user by email and set it to "Active" like this:
> user = User.find_by_email("[USER-EMAIL]") > user.state = "active" > user.save > exit
Replace [USER-EMAIL] with the actual email you're looking for. The user should now be active and ready to login.