I saw a video a few years ago with people speaking into microphones connected to a digital delay attached to headphones they wore. With something like a 200 - 300ms delay most people could only speak a few words before becoming unable to speak intelligibly.

Something like that, with a directional microphone and one of those eerie directional speaker rigs I find in retail stores could be tons of fun for those irritating people who insist on using speaker phone in public.