If I have such things, I who.is the owner, search the abuse mail and mail the administrative contact of the domain to put pressure on the owner of the email. A bit "heavy", but it works.
If it's Hotmail or Gmail, I'm sure you'll find some page somewhere.
If they continue to treathen you, ask your host for the apache logs. Check the IP's and actions, and mail it to the abuse of the domain (generally abuse@domain.ltd). Most of the times, the host will then undertake action. If not, just mail the owner of the domain. If you take a serious layout and very formal language, they'll shut up in no time.