There's an elecrtronic device you can buy that works great for this situation. You plug it in over the mouthpiece. When they call make absolutely sure its the guy first, then hit the button. It sends an ear drum bursting electronic blast over the phone line that will fuck up the guys hearing permanently.
I had a problem with a harrasing abusive caller long ago and got one of these phone blasters. Only had to use it once and he never called again.
