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You can file harassment charges and seek a restraining order against this person to prevent him or his children from following you around, contacting you, etc.
As for the defamatory information he's threatening, there's not an awful lot you can do pre-emptively other than to tell him that he will be defaming your character and that you'll sue for the libel (written) or slander (spoken) if he knowingly makes false statements about you that are passed off as true.
If he doesn't heed your warnings, then you could sue and seek an injunction to stop the dissemination of this material; you could also seek money damages for the impact this has had on your reputation.