If there's a person in my life i don't care for, I don't pay them any Attention. If you do this, they will get bored of trying to talk to you and maybe move on. And if this is online, simply block them.
Calmly and gently say that you are really sorry, but you don't see him in that way. Apologize for leading him on or giving him signs, even if you didn't notice or mean to.
