There's this guy who likes me, but I'm not interested. What should I do?

Pretty obvious from the title, but I've only talked to him once before I was told he "likes" me. The one time that we talked wasn't even a real conversation; it was a greeting, and that was a while back. I didn't believe it, considering I'm pretty average overall (and I only have one class with him), but then I started receiving notes from him and even a gift. There are plenty of girls who aren't as plain-looking as me! It was flattering at first, but it started to become overwhelming (and honestly, creepy)!

He thinks I like him, but I don't, and I'm clueless as to what I should say to him. On one of the notes, I felt as if he was conceited. I thought he was going to ask me out, but the words were arranged as if he was telling me to ask him out.

The notes he passes to me aren't even passed by him; they're passed by people who are nosy enough to actually read it before handing it to me! The content of them are embarrassing as well, though most people would probably disagree and say it's "romantic".

I feel better now that I let that out. I'm sorry if I sound like a horrible person, but how can I like a person who I don't know? How should I explain to him that I don't like him?