How to politely reject random guys who ask for my number?

I'm 18 and work by myself at this kiosk in the mall (I'm not proud of it, but hey I need money). Anyway, at least once every day I work, some guy will start talking to me and eventually ask me for my number. I have a boyfriend, but the way these other guys talk makes it hard for me to bring it up. I've handed out fake numbers before, but I feel kinda bad doing that. What's a polite way to be more honest about my disinterest? Thanks!