Determining Catchability of a Pokemon without wasting a Pokeball

Yes, you can cancel a Pokémon ball throw if you hold the ball still. You simply need to release it, and it will cancel the throw. Though from experience, if you move it slightly while releasing it, it will just drop at your feet. I do this quite often to avoid wasting time trying to catch my 397th of a Pokémon that will require the use of a great ball, or several tries with a Pokéball.