Is it really so hard for the average person to believe that someone wouldn't want their house to be used as a free billboard? I put up a No Soliciting sign and yet some companies still insist on papering my house with their ads. The latest is IFA Country Stores. They go into the bozo bin and I will never do business with them because of this.
CA, I think a No Soliciting sign on one's property is like the Do Not Call list: both are effective only if a company is ethical and has respect for people's privacy. I've had a No Soliciting sign on my front door for years now, and I still get flyers stuck in my security door every so often. They go right into the recycle bin just like the weekly ads that fill up my snail mail box. You really have to wonder about the management of a company that thinks stuffing flyers in doors (or under windshield wipers on cars at the mall) is an effective means of advertising in 2014. My guess is that 95% of the flyers distributed go straight in the trash or recycle bin. Although much of IFA's customer base is probably older and has minimal exposure to the internet, even old farts like me are abandoning print media in favor of the "interweb", to quote Richard Rawlings. Aside from the local yellow pages, the only print media I'm exposed to on a regular basis is a couple of Hemmings publications. Even if I find a business that sounds promising in the yellow pages, I'll usually go online to read the reviews before I ever set foot in the door or make a call for repairs.
Reading your post made me chuckle as I remembered a Dear Abby column I read years ago in the Minneapolis Star-Tribune. The letter writer had a No Soliciting sign on their door and the Jehovah's Witnesses rang the writer's door bell. When the person answered the door, he mentioned the No Soliciting sign to the Jehovah's Witnesses, who replied that they were not soliciting but were "offering witness" to the presence of the Almighty. So the person who wrote to Dear Abby made up another sign to put on his door which read "No Soliciting, No Witnessing, No Kidding!" Perhaps you should try that approach to discourage the solicitors from using your house as a free billboard.
I tried an even more bold experiment once. At one point I included a note saying violators will be charged $500. That didn't work, though I never actually filed the lawsuits. Almost did a few times though. At the very least I think criminal charges of theft by conversion and trespass to chattel might apply (I am not a lawyer.) I admit it's tempting.
I'll say this. It's definitely cost businesses money. After Harmon's pulled that crap I stopped shopping there.