I have to make the phone call this morning. Not fun. I've never done it either.
Blurt out, "So sorry loser! We hired someone better," when he picks up the phone.
Disguise my voice and tell him I'm the boss's secretary.
Email him a photo of my middle finger with our logo on it and let him figure it out.
Tell him he was "Too Good" for the position.
Tell him we were going to hire him but we didn't like how he tied his shoes. Seemed too odd for our company.
Mail him a bottle of scope with our company card and a note, "Better luck next time, Stinky."