How to politely say you don't want to be friends but not too politely because I want them to understand that there was no friendship to begin with.

About 9 months ago this guy started talking to me on tumblr, very clingy and weird. I only replied because he reminded me of my sister who has the same social struggles as he seems to have. We do have some common interests but my god is this person weirding me out. I can't decide if he's creepy or very bad at talking to people, which is why I felt bad and kept replying. The way I see it it's like that one kid you sit next to sometimes in class and have occasional chat with. That's what it is to me. Then I didn't log on for a while and got so many messages about how he wants to talk to me, be there for me (creepy) and talks about how he feels like everyone's leaving him. I think he just wants people to be there for him and has become quite skilled at using the pity card, I hate that. It's annoying and feels kinda mean. Unless he's autistic or a sociopath or something. I don't think he has any friends and never wants to talk about that and I don't wanna ask. I don't want to be his friend either, he creeps me out and has this strange sense of entitlement that shows every time I ignore him. I've ignored him for a few weeks now and the messages keep coming... Should I politely tell him to stop talking to me? I could block him but then he could always make another account. Or I tell him to fuck off because the more messages I get the less remorse I feel.

let reddit decide and I'll do whatever you guys think is best.