Why do people do this?

one of my friends has an brother that is a year older than me. I talk to him and occasionally flirt with him. I found out a few days ago that my friend and her brother tell everyone i like him even after ive straight up told him i don't. It really bugs me cause now all my friends always bug the crap out of me about it and make fun of me. I usually just ignore it and plainly tell them that i dont like him. How can i get people to believe me when i say i dont like him?