It's amazing how many "experts" there are who don't have a life of their own, and so consider it their mission to tell others how to live. Whatever happened to minding one's own business? (Otherwise known as "good manners".)

If you can practice in the mirror, raised eyebrows and a frosty; "I beg your pardon?" repeated as often as necessary should manage to finally make them realize that they are being rude.

Then again, so many boorish people cannot conceive of the fact that it is their manners at fault.