While working in a country with many pushy street vendors, I learned that speaking was absolutely the wrong thing to do if I was not interested. "No, thank you" is seen as opening negotiations. I now give a shake of my head and keep walking. Mall culture in the US is becoming similar, with all the kiosks, sample pushers and sprayers. It feels a little off to me not to respond with, "No, thanks," but the problem has become acute and I don't any more. Sharp "no" shake (some places it is a different gesture) and walk on.