To lower the fan speed, tighten the nut that holds the fan to the horizontal shaft. To prevent the fan from spinning altogether, loop a bungee cord or other tie down around the fan's wire ring and the windmill stand. Can also remove the head by sliding it off of the vertical shaft. All of these would have to take place before the high winds arrive.
Note that our 60-inch windmill head's horizontal shaft can be adjusted so that the fan is parallel to the tail, which will lower the fan speed.