One diamond-shaped standard kite and string
Strips of cloth or bedsheet
One glass bottle
One paper bag
First, buy your kite and string. For this particular job, I prefer a kite with a big picture of Barbie on it--that way, the victim will never see it coming. Make a tail for your kite with the strips of cloth, and douse it in glue. Stick the glass bottle in the paper bag and bash the shit out of it with a hammer until nothing is left but tiny sharp glass crystals. Carefully pour the busted glass all over the kite's tail and let dry.
Now for the fun part! Practice flying your kite until you feel like you have pretty good control. Then run out to the park, and stand next to the asshole with the stunt kite. Naturally he will laugh at your pathetic Barbie kite--but will he be laughing when sweet innocent Barbie uses her gleaming, sharp tail to slash his $200 kite to shreds? And will he be laughing when his most prized possession in the world comes tumbling, tumbling down to earth, landing in a pathetic pile of twisted rubble? No, he won't be laughing. But you and Barbie will. And that's because it's FUNNY.
Warning: Sharp kite tail could accidentally slash someone's eyes, causing blindness, or wrist, causing death.