I initially thought the same and found it was really hard to shake it off. It took me several weeks of frustratingly sticking up with the exercises until I was able to do it.

But I still tend to fall back into it when I'm reading something hard to understand, specially when reading english which is not my native language.

I never consciously inhibited subvocalization, it just must have happened at some point as I got faster and faster at reading. I think it just comes down to speed. I comprehend the word or phrase before I can even begin a subvocalisation and so I'm already onto the next section.