Can't say that I am very happy reading
some of the responses in this thread.
We are not talking about people harrassing you, or trying to ruin your shot. We are talking about another human being trying to engage you in a simple conversation. Not exactly a major crime. Be happy that someone cared enough to even be interested in talking to you.
I guess I am not uberly cool enough where I have gaggles of attractive girls making cheesey innuendos.

But is it so hard to simply respond pleasently when a stranger talks to you? You don't have to cancel your shoot. Every one I have talked with understood that I was in the middle of doing something and they leave after exchanging a few friendly words.
Let's face it, if the stranger is going to steal your gear, they are probably not going to talk to you first.
Like it or not, we do represent the photography community. Do we really want nonphotographers to know that photographers are socially inept jerks? We are, but we don't want everyone to know that. LoL
I just don't see how saying "thank you" in a pleseant manner is difficult. They are not asking to borrow money from you, they are asking about camera that you know you are proud of.