Toxicity of fluoride ion comes from its affinity to Calcium. When it enters the blood stream, it binds ionized Calcium, making it unavailable to support the vital functions of the organism.
Just ONE GRAM can cause a severe paralysis and brain damage. (this is for an adult of average size).

However, its affinity to Calcium can also be used to precipitate fluoride ions in water by adding soluble Calcium salt to it.
Calcium and fluoride ions would then bind together, producing Calcium fluoride, which is practically insoluble.
Very low solubility of Calcium fluoride means a very limited "bioavailability". Poison is thus effectively neutralized.

The easiest way to make a food-grade soluble Calcium salt is by putting eggshells into a container and pouring vinegar in it.

After a day or two, depending on temperature, all vinegar would be consumed (provided there was enough eggshells) and you would get a solution of soluble Calcium salt.

This is a traditional Calcium supplement, which was recommended for pregnant and lactating women to help them preserve their teeth as a result of an increased demand for Calcium.

However, if you decide to use it as a Calcium supplement, then you should keep in mind that Calcium should always be taken with Magnesium, with an optimal ratio being about 1:1.
Most commercial Calcium supplements are sold in combination with Magnesium.

However, even though you can neutralize fluorides this way, it is of little help against other contaminants.

The stuff that is used for water fluoridation is not pharmaceutical grade, often it is unprocessed sediment, which is collected from the wet flue scrubbers in metal smelters and pesticide plants and includes many very dangerous contaminants - arsenic, mercury, cadmium, lead, to name just a few.

Solubility of Calcium fluoride in water at 20°C is 17 parts per million (ppm) or 17 milligrams per liter of water.
This solubility figure refers to a concentrated solution, since this is the only "stable" solubility number.
Numbers below the concentrated solution figure can be anything and are certainly not suitable as a reference point to evaluate solubility.

If the total available pool of fluoride ions can "only" produce water with a concentration of 1ppm, then, after adding enough soluble Calcium salt to overwhelm all the available fluoride ions, there would be no more "unattached" fluoride ions in that water, since Calcium ions would bind a corresponding number of fluoride ions, and if the number of Calcium ions is higher, they would bind all those fluoride ions and there would be no residual solubility from Calcium fluoride.

This is described at http://www.deadwater.info/neutral.htm