There's no way he's not going to be hurt if he's already that into you. That's not your concern. If his world view doesn't encompass not getting too into a girl without first making sure she is also or could be into him, frankly he did it to himself. It's his stinkin' thinkin' that caused him the pain, not anything you did.
Be completely direct and leave
no opening for misinterpretation or misplaced hope. What Marcie said to say...
| "You know what? I think you're a really great guy, and I'd like to spend some time with you, but you need to know that this isn't going to be going anywhere romantically." |
...is the wrong way to go because I
assure you Mr. Clingy will interpret "I'd like to spend some time with you" as meaning there is hope for him. He'll cling to that like a dying man to a life preserver. (Probably ditto for "really great guy.")
Make it absolutely clear that there is no hope for any kind of relationship. If the guy still somehow doesn't get it, cut him off completely and refuse to have anything to do with him.
The sooner he gets the message and finds a new target for his "affection" the better it will be for everyone. That
is the kindest thing you can do for him.