I can't remember if "No Soliciting" signs slowed me down or not when I was a missionary. So probably I just ignored them or not according to my mood at the time.
I designed this one in my sleep the other night. I think it would take maybe $1000 in set up fees to get a sign making company to create yard signs of these. But the cool thing would be to sell these door to door. It could be a genius marketing plan. Everyone you talk to will already be thinking: "Geeez I hate sales people. I ought to get a sign that says..." and then you hand them the product.