When dental fluorosis starts, is it possible to stop it?

Not reversible. The condition is not reversible, and treatment is limited to a few cosmetic dental procedures (microabrasion, veneers, etc).
Nope. Unfortunatley, fluorosis causes the changes as the tooth is developing under the gum. By the time it erupts, the changes have already occured, as another poster suggests, make sure that Fluoride is not over-supplemented. Some hidden sources are things like juice boxes and drinks reconstituted with city 'fluoridated' water which should be taken into account in addition to your own drinking water.
Yes and no. Once a developing tooth has an area of enamel fluorosis occur it is usually there for life. To stop fluorosis from occurring you need to reduce the uptake of Fluoride to optimal levels. Not enough Fluoride during tooth development makes tooth enamel less resistant to acid attack. Too much Fluoride does the same and causes a defect in the enamel via color change or texture. 0.7 ppm fl is optimal.