Thank you for posting this Mulan. I was speaking to a few friends who are Police Officers and my cousin who practices law. They told me specifically what could happen and why it is better to have a No Trespassing Sign than a no solicting one.
You can press charges, they can be fined, arrested and even serve jail time for repeat offenses. If continually disregarded you can even sue for harrasment. The sign gives Officers more bite to the offender. They can ask, "Didn't you read the sign or see the sign?" If it is completely visable in several places then there is no excuse for the trespass.
Regards,
X.
Ps: I can imagine people putting up the signs and barbwire... lol