I met my junior high school classmates by chance when I was in primary school. He always sends me messages and calls, and says he likes me. I was annoyed by him.

You don't need to feel guilty, because you don't like him, you have to explain to him, and you can't interfere with his thoughts, so it is his business that he likes you, and it is your business that you don't like him. He keeps texting you. You can delete his message without reading it, and you don't need to feel guilty at all. If you feel guilty, he will think you are sorry for him, so he will pester you more.