Tim's idea of a closed circuit TV is the best approach, and something I've been thinking of for a long time. By far the best way to interact with anyone you didn't want around. You'd want to couple it with a computerized voice interface that made it sound, at least, that they were dealing with a machine. "State your full name" "Face the camera" "Turn face left" "Turn face right" "Hold ID up to camera" "State business" and at some point a speaker off to the side or behind them could emit the distinct sound of a 12ga pump action being being worked, followed by a: "Leave property now"