How to stop my cousin from being so annoying?

My cousins mom and dad went on vacation, hes staying with me and my family. So basically, (hes 8, I'm 14) He'll get into all my stuff, I bought a super expensive air conditioner for my room, (400$, and I'm 14 so that money meant alot) and he throws a baseball at it and cracks it. I had a nice mountain bike, I BOUGHT, and he goes out and drives it around runs into my moms car and cracks the running board, and makes the chain get messed up, so we got a new one and fixed it, and he broke it AGAIN. He was spinning around in my computer chair and kicked my dogs leg while he was spinning, and my dog is limping, and he tipped the chair over backwards and cracked my computer screen, and he keeps going on these random sights on my kindle, he opened my old laptop bag and was taking the chargers and cords out and stuff. He opened my old box of toys, and threw them all outside. Help BEFORE I KILL MY FLIPPING COUSIN…

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