I generally walk away and just ignore them, though there was this one time I was walking (yeah, I'm tough) home from the pharmacy when I was sick with a stomach virus, since it was just around the corner and I needed some Pepto. I had just taken some medicine so I was feeling alright, when some guy from high school ran into me and started talking. He was an antagonistic guy, sort of a jerk, and he was acting like he was my best friend. He had also broken my best friend's heart, so I didn't have a high opinion of him. When he was waiting for a response, I just stared at him in silence, and then I wound up throwing up on his shoe. That was the best timing I have ever had.