How can I make fizz free and alcohol free mojito?
Harms? Meh, that's silliness. You can make your own fizzy water if you're so inclined. Sodamakers.

If you really don't want fizzy, then just substitute still water in any of the recipes in a search for Virgin Mojitos. Choose recipes that use soda water, club soda or seltzer water, not ginger ale. Ginger ale adds flavor, you want recipes that stand without that flavor.  

EDIT: Just to be clear; soda water, club soda or seltzer water are all just water, injected with carbon dioxide to make bubbles (carbonation). So by substituting regular water for soda water, club soda or seltzer water all you're losing is the "fizzy". (Club soda may have a tiny amount of something like baking soda to neutralize the decrease in PH caused by the carbonation, Slate gets further into it, but for all practical purposes, they are all just bubble water.)