I don't bother writing to them, just return all mail not known at this address. I also have a 'Private by Appointment Only' plaque on my door and Trespass Notice (they're easy to find on the net but I can sent you one if you'd like?). If anyone comes to the door I haven't invited, I ask if they have an appointment. They say no. I hand them my schedule fee, say 'have a lovely day', then shut the door. They wander off confused and perplexed every time & don't return.