What would happen if a plane crashed in a farmer's field and the farmer is horrible and refuses to let emergency services onto their land and then issue trespass notices to all on the plane?

Well, that farmer would make himself very unpopular very quickly, especially if people need assistance. As well as perverting the cause of justice he could also be answerable to manslaughter charges and would certainly need to be all clued up on international law. There’d be quite a delay in getting any amount of compensation owing to him as the cost of damage to human life would have to be calculated first. He must also own quite a fortress if he’s successfully able to prevent other agencies from entering his property. And is the farmer actually going to walk down the aisle of the plane handing out notices to each seated individual? Or maybe he’s lazy and just puts one on the front of each door? He’ll certainly save in print costs and will assume that people exiting the plane will return there and they’ll see the notice on their way back in.