Assuming he gets hurt by a rejection, which seems likely, he gets hurt by the rejection itself, not by the delivery. The only way not to hurt him, not to reject him, is to accept his offer, when it comes.

It is not easy to reject someone, especially when you are suddenly put in that situation. The person doing the approach can at least be prepared. But you have to do it as an adult, as a mature strong person. Just as the person doing the approach is making himself, or herself, vulnerable, there is no way for the person doing the rejection to take no risks.
And yes, if you are truly not interested, you have to crush that person's hopes and fantasies to get together someday. Because if you haven't crushed those feelings, you haven't rejected that person.

Therefore, there is little room for niceties while doing the rejection. Whatever tension or adversity there may be after a rejection, you amend those later. You don't do it during the rejection, or you give off mixed signals.
Talk to that person that person a few days later to clear the air. They can't do it as they just approached you and didn't get a 'yes'.

Now, all that I say ignores the idea that ther's crazy, vengeful, immature, undisciplined people out there, which there probably are.