Say this:
"This thing you're feeling, I'm not feeling it, and that is not subject to change. Please drop it before it gets really awkward. I don't mean to insult you, and I'm sure you can find someone else who'll respond more favorably to your advances, but that's not me, so please, move along. You're barking up the wrong tree, and there's a whole forest out there for you to bark at."
You have to be direct. This guy's obviously not on the same page as you are, and you have to flip it for him, because he's likely in a place where he thinks you're "playing hard to get", and you might have to assert with absolute clarity that there is no playing going on in order for him to let go of the idea. And if that doesn't stop him, report him, because at that point it's sexual harassment.