There are many adjectives that are used to apostrophise a person stubborn: pigheaded, headstrong... but the phrase that surely better represents person who falls into this category comes directly from popular folklore with the sentence: You are as thick as two short planks!.
The really bad thing for these people is that they realize rarely to be stubborn, in these cases there is nothing better than to propose a Stubbornness Test!
Few questions and a verdict that leaves no doubt!