How about a solenoid that engages when the fan turns off ...mechanically

it

would be a friction clutch as Jeff mentioned or a band around a hub like

a go

kart brake.

Archie wrote:

These are commonly used on fractional HP motors,
all the way up to dual disc brakes for larger applications.
 Due to the pressures attempting to rotate the blades,
perhaps something light and positive may be the best solution.
Perhaps a simple mechanical cable stop applying pressure
against the hub, or...............
Archie

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