The No Trespassing sign and letting them know you are an apostate should work. However, unfortunately, some of them still insist on being stupid and calling anyways. You let them know you're not interested, they are incited to keep calling to harass you. You let them know you are apostate, they continue to harass you. You call the cops, often the witlesses are gone by the time the cops show up only to return the next day or a few days later.
One trick that might work, should it come to that, is to get a Ouija board. Next time they call after having been warned not to, you pull it out and suggest they join for a session of Devil worship. It helps if you have some heavy metal music going and the place is decorated for Halloween inside. Something needs to be done to teach those cockroach witlesses that they are not to go on calling on people after such people have made it plain and clear that the witlesses are not welcome and neither are their teachings. They are obviously not going to "save" a life since there is no way you are going back to that cancer.