Great question — answering as the UphillTech team:
This is called goaltending and is prohibited. In basketball, if you block a shot after it's started descending, it is ruled to be goaltending and the basket is counted automatically . There are several other circumstance s in which goal tending is called as well, but they are out of scope for this question.
Full answer by wax eagle: https://sports.stackexchange.com/a/5547 (CC BY-SA 3.0).
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