I hope after the ball hit him he didn't move at all and just stayed there with his arms crossed as if nothing had happened. If you can, that's the best way to go about this situation. If people are laughing and they see that you didn't react at all, they'll start to question whether what they saw was real or not. "Did I really see that guy get hit in the face with a basketball, or am I going crazy?"
When I was in high school I was hanging out in the library and someone threw a full water bottle at my face. I just sat there and kept studying as if nothing happened. I consider it a victory. A few seconds later they threw another water bottle. Then another one and another one. They kept throwing them at me until I was under a mountain of water bottles.