Replace the NO SOLICITING sign with a big honking NO TRESPASSING sign, taped to the inside of the glass of your storm door. If anyone violates the no trespass, CALL THE POLICE. Tell them that a sales person or whoever violated a posted no trespass. They won't do it again! (If you think I am kidding, I'm not. Our neighborhood is bombarded with sales people. But the only ones I have had to actually call the cops on are the Verizon doofuses. They are RELENTLESS)