A sign matters.
Criminal trespass is defined as "knowingly enters or remains unlawfully in or upon premises of another." RCW 9A.52.080. So the two typical questions a jury would have to answer is whether or not the entry or remainder was "unlawful"--i.e. did the person entering have lawful authority to be on your property--and whether the unlawful entry or remainder was "knowing." The "unlawful" bit usually has to do with whether permission, implied or explicit, was granted. [It may also have to do with other things; a police officer with a valid search warrant for your property is not entering unlawfully, even if you tell him to leave.]
But in your situation, you've got a private residence, where a person rarely has implied permission. [Contrast with a publicly-accessible commercial parking lot during usual business operating hours.] So unless these people were told by you (or someone else with a right to be on your property, like a tenant) they're welcome, their presence is probably unlawful.
The rub of your question, though, gets to the "knowledge" requirement. A person can always argue to the jury he "reasonably believed that the owner of the premises, or other person empowered to license access thereto, would have licensed him to enter or remain." See RCW 9A.52.090. And if there's a big sign saying "no trespassing" or "no solicitors" or the like, the likelihood a jury would believe he didn't know he wasn't allowed to enter or remain goes down sharply.
Will the sign make them stop at the driveway and not enter? Perhaps. But if it doesn't, there may be more evidence for the prosecutor, and that may make them more likely to pursue a case against those that trespass on your property.
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You can post your property. Some folks have a right to enter, for example, the meter reader. Religious missionaries are probably not going to get high police priority. We have had scam solicitors here who looked very suspicious to me, and I ordered them off. The dog has ordered others off. I suggest the repo guys are approaching harassment. Letters are better than calls.
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