If you're from the same class which means you knew each other previously, so when they directly ask you out tell to them with clear language that you currently aren't interested. Don't be rude, ambiguous, vague or anything. And next time when you meet a new person don't make him feel like you are interested. Even talking to him makes the man more hopeful. So don't talk to them unless it's work or education. If it is a random person who you don't know before and only met him online through telegram then you can block him but do so before you talk to much and also say "goodbye" to them or say "I'm not going to talk to you cause I don't know you, so I'm blocking you" before you block