Me: "Don't put your upper blocking hand directly in front of your forehead."
Student: "I'm not."
Me: "Look in the mirror; your hand is directly in front of your forehead."
Student: "No, it's not."
Me: [lightly strike's students hand, which bounces off his forehead].
Student: "Ow."
Me: "What just happened."
Student: "My hand hit me in the forehead."
Me: "Hmmm. Wonder how that happened, since your hand is not in front of your forehead."
I get similar students to that when I'm coaching diving. They insist they are doing the technique a certain way even though it's clear they are not. It gets to the point where I have to film them doing the movement and play it back to them in slow motion before they'll believe me.
On a side note, I have another piece of studenting advice: Listen and be honest. It really annoys me when I have a student who does a technique wrong, and when I ask them about it, instead of thinking about it and giving me an honest answer, they just say what they think I want to hear. For example, a student doesn't swing their arms up high enough on the take-off for a dive. Conversation follows:
me: "did you get your arms all the way through to your ears on take-off?
student: [instantly] "no"
me: "do it again then"
Student does the move again and this time gets their arms all the way through correctly.
me: "did you get your arms all the through?"
student: [hesitantly] "no"
me: "why do you think that?"
student: "because you wouldn't ask me about it unless I did it wrong"
me: "You did it right that time. Stop just saying what you think I want to hear, and think"