If the only way they can die is to have their head cut off then why don't they have adamantium plating surgically attached to their neck and upper torso?
That would work like steel-toed boots do. Fine if you drop a hammer but anything heavy will cut off your toes. A 30 mph traffic accident becomes a beheading.
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Donny: Are they gonna hurt us, Walter?
Walter: No, Donny. These men are cowards.