How can I keep girls from flirting with me?

I'm a student and not interested in dating. I've already had friendships fall through because the girl wanted to be something more. I'm unfortunately good looking (I don't say this to brag; my life would be simpler if I was ugly) and an outgoing, humorous affect that seems to make girls think I'm flirting with them. Changing my social modus operandi feels like an overreaction, and I don't want to open every interaction with "I don't want to date you so stay back hoe."

How can I keep girls from flirting with me?
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