My solution to this
is to wear an obvious set of headphones- not subtle little ear buds, but visible headphones. If people can see them, they know you can't hear them or are at least not listening, and usually they'll leave you alone.

If they don't, you make a show of removing them and asking them to repeat themselves. It doesn't take long for people to get the hint, unless they're being deliberately rude.
( The Resident Loon Not a demographic. Do not measure., Fri 12 Sep 2008, 13:05, Reply)