It's a learned behavior that's become automatic. You're not "choosing" to block in the way you're thinking about it. But you can work to undo the behavior. It's not a core stuttering behavior of disfluency it's an avoidance behavior. There's no magically force causing your vocal chords to lock up. You do it because at some point you learned stuttering is unacceptable, at least in certain situations, and your body/brain goes into a strong approach-avoidance conflict and you block. You want to speak, but you don't want to stutter. So you approach the word, but you feel you will stutter and your body has learned not to let they happen. So you block. You can unlearn his behavior. That's what I mean by choice. It's not a forgone conclusion you don't have any control over. It only feels like you don't have any control over it because at this stage, you don't.