Friendships require two consenting people. Someone can’t be your friend unless you also want to be their friend. Friendship is a relationship and it has to be mutual.
Some people do not understand this. Some people want to think of themselves as your friends, and don’t care what you want.
In effect, people who do this are treating you as an imaginary friend. They don’t want *you*. They want an imaginary different person who wants to be their close friend. (And, they probably want a number of other differences, too.)
If they wanted you, if they were interested in friendship with the person you actually are, they’d respect it when you said no.
You can’t usually stop someone from perceiving you as an imaginary friend, but you don’t owe them your cooperation, either. It’s ok to ignore them. It’s ok to refuse to listen to lectures on why you’re being a bad friend. You don’t have to give them a chance and you don’t have to convince them that you’re right to distance yourself. You don’t owe it to anyone to help them pretend you’re their friend.
You can’t stop them from thinking whatever they want to think about you. If they send you lots of email. Or letters. Just don’t read them. Because they’re interacting with an imaginary person. Not you. And the real you doesn’t have to play along.