The reason we discarded the ball idea was because it means we are limited to 3 intercepting shots (you can only possess 3 balls), whereas the air system would have unlimited ammunition (disregarding battery life etc.), and like Ninja_Bait said, we would go work for NASA
A cannon may not be effective enough given the pneumatic parts allowed (valve is too small i hear and the max psi is too low), fans will be hard to make large and safe while effective enough, and leaf blowers have massive draw on battery.
One idea we had like what Spen.M.P. suggested of an air knife was to have an industrial blower that they use for clearing milling machines and the like, for a concentrated stream.
And like davidthefat, Nuttyman54, and Ninja_Bait said, the difficulty of tracking a ball traveling at more than 10 fps with a lagging camera would be incredibly difficult, and you're paying the opportunity cost of the best motors to power a limited range defensive system that may not be able to protect the top hoops, and the amount of training required for drivers could be more than if you trained them to just shoot baskets.
One way to alleviate this is to make a system that shoots a wall of concentrated air up that is say 5 feet wide, with 3 rotating air cylinders like an autocannon that recharge when not firing to increase the rate of fire.
But this goes back to the limitations of the pneumatics allowed.