They are right: fluoride ion is a natural compound, just like every bacterial and viral disease is perfectly "natural" - plague, leprosy, syphilis, all types of hepatitis etc etc.
It is true, there are certain regions on Earth where calcium fluoride rocks predominate, there is not enough soluble calcium from other sources and that is why there are "naturally occurring fluorides" in ground waters in those regions.
Calcium fluoride can produce a concentration of fluorides of 17 ppm, which is dangerously high level. This refers to a concentrated solution, when reserves of undissolved calcium fluoride are available in the same pool of water and there is no other source of calcium ions to bind fluorides.
Punjab, as well as some other areas of India are good examples. There is lots of bone disease among the population.

IMHO it is pointless to talk to those who get paid to push that poison on people, the only realistic way I see is education at the grass-roots level.

There are quite a few quotes from medical sources on the site http://www.deadWater.info
maybe the people in your area can band together and print flyers using those quotes or make an Internet site.

In the meantime, adding a soluble calcium salt to drinking water is a reliable way to neutralize fluoride, but, of course, it does not "neutralize" arsenic, mercury, cadmium, lead, which is in the wet scrubbers residue, which is used for fluoridation.

HTH: Oleg