E.B.
Use some kind of color code. Get sturdy poster board or similar durable fabric, foam board etc., and cut out a star and a stop sign shape. Make the star green and stop sign shape red. Get your son to help decorate them (glitter or stickers or designs). Teach your son and the neighbors that when the green star is on the door or in the window (somewhere visible where a child can see it), he can invite them in or they may ring the doorbell. If they see the red shape, that means "no friends inside right now" or "we're having family time or doing chores, please do not ring our bell or knock on our door". Role play with your son. Have someone pretend to be the kids outside, and show your son how to check whether the star is posted or the stop sign. And also, teach him how to politely invite someone in when the star is visible. Be very very clear, in simple blunt terms with the neighbor kids. Don't say "we'd really rather that you avoid..." Just say "When you see the stop sign, that means no door bell, no knocking, and our son is not able to play then, outside or inside. Do not come to our door or play in our yard when you see that red sign. You will see this bright green star when it is neighborhood play time". It might be a pain at first, but it will be less painful than disconnecting the doorbell!