How do you let someone know that you don’t want to have a friendship with them?

I feel awful because I see myself in them. I was a painfully shy and socially awkward child, so I understand the pain, shame, and sadness that comes with being alone. Especially when everyone around you seems to have a friend group.  However, I think I was self-aware enough to not be completely cut off from the typical primary-school social life, which this person does not appear to have any awareness (respectfully.) I suppose I was just hyperaware with what others thought of me since I was afraid of being ostracized yada yada.

Anyway, the person I don’t want to be friends with… has been making me more uncomfortable lately. I think I messed up by being too friendly to them. We play a sport together in high school, and because we see each other every day for many hours, I make small talk and try to include them because I saw they literally don’t have any friends. However, they keep following me almost everywhere. Even my friend group, with whom they rarely interact with. They cross my boundaries, such as touching me or intruding on personal moments with my friends, and it bothers me. But I don’t want to feel like that because they’re just trying to be a part of something, a feeling that I’ve understood before. Still, it’s not fair to cross my own or even my friends’ boundaries.

Also, their mother helps with the tournaments and activities, so I’m worried that if I try to set strict boundaries, they’ll tell their mom, which will spread to the other parents, causing them to treat me differently.

I’m not sure what to say or how to act. I don’t want to crush their heart, and I hate seeing them be alone all the time. But I just don’t think we are compatible enough to be friends.