What you might consider is using the member management tool in the control panel. With that you could specify a search for users that haven't logged in since a certain date and have 0 posts, that might catch a lot of those users you're looking for.
Thanks Rick for the feedback.
Unfortunately we often get users that go quiet for over a year and then come back. We can't have those users deleted, even if they have never posted.
Perhaps a cool feature would be a 'prune bogus users'. Essentially, we want to remove all users that never officially finished their signup.
