This guy gets his kicks by provoking a reaction from you. If you have caller ID enabled, simply ignore his calls, or answer the phone but leave it off the hook and walk away.
If no caller ID is available, then file a complaint with the SAP, mention that you're being harassed with threatening phone calls, and get the ball rolling. It might take a while, but he'll get nailed in the end.
Also document the times, dates, etc. that you received the calls, as well as the contents thereof - it'll be helpful if it gets to court. Not sure if recording the conversations would be legal or admissible in court without him being notified that the call is being recorded, but you could scare him off by telling him beforehand that all calls are being recorded and monitored - and achieve the same thing in the end, a stop to these harassing phone calls.
Finally, if you can hold your temper in check, try reason with him calmly. Maybe he's upset about something, and is displaying his displeasure inappropriately vis-a-vis the threatening phone calls. If it's not that, and he's really being an idiot, at least you know you've tried, and the records will back you up on that.
Just my 2c, VAT included.