I never found a fool proof way to miss the time shot. The way I eventually did it was to turn to the left so that the Kinect was facing the right side of the body.
Then I would push my hands straight ahead of me and just enough to the right that the kinect would sense it as a shot.
This still didn't work every time but it seemed to work far more often then when I just tried to screw up my shot while still facing the kinect (made the vast majority of these) and for some reason when I turned to the right to do the same thing.
I have a question about 3 Point Contest. Towards the end there is a challenge for "Scoring 40 points without time bonus." How in the hell do you do that? I try to miss the 5th shot but I always seem to make it in.