how do I get someone to stop flirting with me when I have a bf?!

Sarah • 17•💕2-6-2015💕•HHS•
So there's a boy in my math class who is always flirting with me and tries asking me out once or twice a week but I have a boy friend and this kid knows I do. What do I do because I'm not breaking up with my boyfriend but this is just getting really annoying now!  (I'm in 11th grade if that info does anything) Help please!!!