This week at the book study, at all congregations in the U.S., a letter was read saying if you go to a house or property with a "No Trespassing" sign, you must not enter, or knock on the door. You have to leave immediately.
A sign saying "No Solicitors" is too ambiguous, and they are to use their own judgment, and don't have to knock.
If it says "No Jehovah's Witnesses", you also must leave. Don't try to talk to them.
I thought this was pretty interesting. There must be some lawsuits out there.