Since our town requires a permit to go door to door, if the bell rings and I see a person on the porch without an ID tag prominantly displayed, I call 911 pronto. The last two times I did this, they had the person in the back seat of the squad car within minutes, usually by the time they got to the next house. I'm sure they just checked their ID, advised them that soliciting door to door without a permit was not allowed and drove them to wherever they belonged.
The permit policy applies to everyone. local Cub Scouts, Little League, Cheerleaders, petty politicians, Muslims for Peace, Evangelists, bag draggers, whoever. No mercy. You want to ring bells on our block without registering with the police, you're going to get a tour of the back seat of the squad. I love it.