How about an adult that hasn't been to a dentist in over 25 years?

Get an air horn, then answer them.... I've also found queuing a particularly loud, and screamy clip from a B horror movie helps decrease these calls.

Edit: I have the track "Friday The 13th" by Figure saved, and set to the point where it goes "Jason Voorhees is dead..." and a very abrasive "drop" at a moments notice, I answer spam calls, and often play this.
My spam calls have decreased.