I'm not the expert here, but I would think a good fan clutch would keep the fan spinning and be launching any object used in an attempt to halt it. Though I've not ever attempted to stop a fan from rotating, I'm against hospital visits for things being launched into my body by the fan.
The viscous coupling is designed to let the fan slip, thats what reduces the load on the engine. If I remember rightly the fan should slip at around 4000rpm so even if the engine is doing 6000rpm, the fan only turns at 4000rpm. It should be possible to stop the fan with a rolled up newspaper or similar until the engine temperature starts to increase above normal operating temperature. At higher temps there is a bi-metallic spring in the centre of the fan clutch which expands and locks the fan up harder, increasing the throughput of air.
It took a good many years and a lot of wading to kill my fan clutch, I would suggest removing the radiator and ensuring that the fins are clear, my only overheating problems were caused by a clogged radiator and an airlock after replacing it. If you do a lot of mudding then ditch the fan shroud as it makes it impossible to clean the radiator sufficiently in-situ.