How do i keep a co worker from bothering me?

I've been working at my job for 3 months now as a teacher istant and there's been a janitor who has been contstantly bothering me telling people that we go together and embarring me in front of students and other staff members. He's asked me out twice and i've told him i dont like him and im not interested in him. He's twice my age but he acts like he's 16 and i've been ignoring him for the past month but it doesnt seem to be working. It's embarring as hell! What should I do?