Break it off. You don't have to say anything detailed or specific, just say that you don't think you're compatible and don't want to see him anymore.

My ex did this all the time with waitresses. They'd hit on him in front of me at dinner when I'm gorgeous and none of them were even remotely comparable. It turns out he was flirting with them behind my back and knew them intimately, as in he was hooking up with them behind my back. Most of these types of server girls are very low quality people in general; much too dumb to get real jobs. They go out with guys even if they're spoken for. The last time he did it was our engagement dinner. I walked up and left the restaurant, for him to chase after me but on what should've been the most romantic dinner of my life, he was up to his usual tactics.