If placed in that situation I will also step back, put my purse in front of me or suddently turn away to greet someone else. Most people mean well but when I don't know them or if I'm not receptive, I decide what happens next. Sometimes when it's someone that I'm meeting for the first time I will extend my hand if I see an oncoming hugger. That sends a nice welcome with a good boundary. That usually takes care of it without having to confront-but I would have no problem speaking up if anyone were to persist in trying to get that close when I'm not comforable with that.