fuck that nonsense. do it the right way.
take your doorbell and bring AC power leads right up through the plastic. leave both ends exposed but try to camouflage them, and make sure they do not touch each other or anything conductive. if you do it right, you can put it on a switch and plug it in to the wall or something.
when they go to press it they'll get one hell of a shock. and they'll NEVER COME BACK TO YOUR HOUSE AGAIN.
~ dan ~
alternatively, you could put a digital camera or something similar in the peephole and use the doorbell to snap a picture?
but fuck being nice.
or do like i would. load up a shotgun with some rock salt and lie in wait. when they get to the door all giggly, open it and they'll be facing down two big fucking barrels ON YOUR PROPERTY. "if you were smart, you'd go home and NEVER SET FOOT ON THIS PROPERTY AGAIN." it's a bit redneck style, but here in the states it's perfectly legal(ish).