How about a sign that says "NO SOLICITORS TRESPASSERS WILL BE SHOT", beside a laminated used pistol target... the use being the object of a couple of loads of the buckshot of your choice from about 7 yards. Seriously I can't believe that the SCOTUS would rule that you must allow folks onto your property that you don't want there. Apartment dwellers may have a problem though, unless the entire building or complex is declared off limits to uninvited salesmen, since the common/public area pretty well extends right to your door. Do you have a link to the decision, or at least the name of the case?
Now That is creating a 'hostile workplace' for door-to-door salesmen!