Don't delete users that register but never log in.
[Marked not implemented 2017-12-13]
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Or do, but after much much longer. Also send them a lot of emails and newsletters. Saying no to newsletters can be done when logged in.
Don't do this. They may not be in control of their emails, we shouldn't send spam to bots. So just delete dem after a week or so if they have not logged in.