That *****. Try driving around in a Hummer and getting flipped-off what seems like every few minutes....In my experience, there are two civilized approaches one can take (besides ignoring) that seem to resolve the issue:
1) Following and confronting them seems to work as they get an "oh darn" look on their face - two downsides to this approach is that it takes time and your passenger might get a lil' scared/worried.
2) It seems to really chap their hide when you get a big cheesy grin and give 'em a big ol' arm wave.
Some people are just jerks.