Dealing with clingy people

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Hi anon, so here's the story.
I'm in boarding school at a prestigious school and there's this guy in my class that confessed to me a few weeks ago, I rejected him while telling him that I was fine with that, that I didn't really care and that I just wanted him to act to normally afterwards and not think too much about me, the problem is, he's now stalking me even more frequently than before. He's joined our group of friends and is constantly seeking ways to spend time with me no matter what, keeps getting into my room asking me for silly things he could ask anyone else about and so on. The thing is that I'm getting tired of having him constantly following me around and I don't know how I can tell him to stop acting like that without being rude to him. I don't want him to start avoiding me, I just want him to stop stalking me.

How do I do? What do I need to tell him? We're both adults and he's a smart boy so I can't even start to understand how he just can't let me go, it's like we don't really even know each other.