How to stop a stalker?

There is a guy who was my friend before, but he became a bit obsessive as time went on and I broke up with him. He tried to insist on continuing to be my friend, but I stuck with my decision.

And I think he is stalking me. Last Friday, I was walking into the bathroom and I saw him right behind me (he turned away). I keep having sudden encounters with him.

The other day after class I was leaving school and I see in my bike basket a letter from him and a chocolate.

Every time I look at him, I see him watching me.

He was nice to me, and I don't really fear him, it's simply weird and uncomfortable. He is indeed obsessed with me. How can I make him stop? I don't want to hurt his feelings either...
How to stop a stalker?
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