Not being an aerodynamicist, my untrained mind would imagine there to be less drag if air was allowed to go through a rotating fan, than if the fan was held fixed in the airflow - presenting a huge “airbrake”. As several have said, turning the fan and engine and gearbox will take power to do, but would that be more power than the equivalent “airbrake” would absorb though?
I suppose that a ‘plug’ of nearly stagnant air might build up in the intake in front of a non moving fan, which might produce a virtual fairing, around which the airstream could flow relatively smoothly?
Wasn’t there a once a four engined jet from somewhere that landed at Heathrow with one engine fan held immobile with ratchet straps - or have I imagined that?