How to tell someone that u don’t like them?

So like there’s this girl I was talking to for a short while and I met one of her “friends”. Which is my one of my friend’s ex’s, so like I knew he was a bit weird but I didn’t think much of it. Well he gets my insta and send me good morning texts and I don’t really like him like that, he’s just a friend to me, he also was upset that I was hanging out with my friend (his ex) at lunch instead of him, he just stood there, like I’m not gonna tell him to leave but it was really awkward even for someone who’s whole life is just awkward situations. Then when the bell rung he followed me class talking about how he couldn’t remember what class it was and then started slapping the shit of my back when I said I hoped he could figure it out. It hurt a lot and pmo so bad. He constantly trying to be close to me and likes me in some way but idk how to tell him I don’t feel the same. he doesn’t seem to pick up on social cues, I want to make it obvious that I don’t like him like that. I just wanted a normal conversation with someone turned into a weird new admirer ig. Idk sorry about the rant ik no one will read this :b