Tell them "Leave me alone. I have a boyfriend. Stop flirting with me. I don't like you." If it doesn't work then maybe tell your boyfriend. My ex (when I was dating him) was a 6'3" all-muscle marine ROTC monster. And this short nerdy kid came up to me and was like "I am going to break you and your bf up. I promise you that." and I told my bf and it eventually stopped after he said something to the kid.
