What are the best words to use to (kindly) let a man know that I'm not interested?

I've always had an issue with this. It breaks my heart to hurt other people. Either I hurt them, or they think I'm not serious and then they try harder.
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I'm not sure if this will change anyone's answer, but.. what if you were friends with this person for years? Most of the men that I'm around, (I mainly have male friends) are super close friends. One of my male friends told me that "no man" wants to just be friends with me or any other woman. I don't want to lose any friendships, but.. I also don't want to be with them romantically. If it was you, how would you want to be told?
What are the best words to use to (kindly) let a man know that I'm not interested?
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