The intercom sounds like the best idea to me. You are never going to stop the people with a sign. They all figure why not ask what's the worst that could happen you will say screw off. Plus maybe there is some kind of intercom that wouldn't make a loud noise and wake up a baby.

I don't like people trying to sell me stuff either but I think a municipal ban is going too far. All I get here are parents with their kids trying to sell fund raising stuff for some after school activity. I figure I should be nice I will have a kid some day that needs to sell some crap.