I hated that. My passive-aggressive response was usually to stare at them until they shut-up. Non-confrontation all the way!
I kept giving them dirty looks but to no avail! So I posted a quick blog to vent, and also with a smidgen of hope they might glance over and notice I was complaining about them =-)
Here's my old roommate who completely agrees with you: http://missnemesis.blogspot.com/2005/12/will-you-shut-your-stupid-mouth-face.html
I love comments, and I love to respond to them personally! Is your email activated in your Blogger Profile? This is the way I respond to readers. You can create a blogger profile, even if you're not a blogger!