Basically, it's a tunnel from the house to a hidden doghouse at the edge of the lawn, obscured by bushes or some other camoflage so nobody can suspect a dog is there. Simpy post a sign on your door warning that a dog will attack if you knock, and train your dog to attack if someone knocks (tell anybody who you want to come in to use the hidden doorbell). They'll never see 'im coming, because the dog will come out behind them.

The dog would (preferrably) be trained not to kill, just to rough 'em up a bit then let them go, so the newly-educated soliciter can warn his colleagues to heed the signs.