The old style water pump windmills had a wind-speed trigger gidge rigged up - if a hurricane came up the tail had an over-center spring and folded over, and the tail vane directed the fan wheel aim
90-degrees to the oncoming wind. That tended to stop the wheel.After the storm you climbed up and reset it by hand IIRC.
The aluminum disc magnetic hysteresis brake (mentioned elsewhere) would work to keep them from spinning to destruction speed in normal stiff breezes, but even that has it's limits.
You do NOT want the unloaded windmill fan wheels going fast enough to throw off a fan blade, car radiator fan blades coming loose at high RPM's have been known to punch nice crescent shaped holes through steel car hoods. If you are in the way they can maim or kill.
You can put a disc brake pad on the side of the tail boom, so when the tail triggers over the pad contacts the hysteresis flywheel.
Myself, I'd get real wind generator windmills made (with the old style multi-blade wheels instead of a three-blade propeller) and rig them up with an inverter to feed into the power grid, then you decorate them up to look like old-style farm windmills. The credit on the power bills can easily pay the water bill and the park maintenance costs...
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