Is there a command to stop fire damage in minecraft?
Players in creative lose the fire effect immediately after leaving fire. If you toggle the players you teleport into creative immediately, i.e. /gamemode 1 @a, then back to survival (/gamemode 0 @a) then the fire effects will go out.
There's no specific command for it, but there is probably some commandblock contraption that can put the fire out.
Optional: give the player fire resistant (potion effect)